Drop requirement for a CLA

The Mbed TLS project no longer requires a CLA. Contributions from now on
must be made under both Apache-2.0 AND GPL-2.0-or-later licenses, to enable
LTS (Long Term Support) branches of the software to continue to be provided
under either Apache-2.0 OR GPL-2.0-or-later. Contributors must accept the
terms of the Developer Certificate of Origin (DCO) by adding a Signed-off-by:
line to each commit message.

The software on the development branch continues to be provided under
Apache-2.0.

Update README.md, CONTRIBUTING.md and pull_request_template.md to explain
the new licensing model. Add a copy of the DCO to the project.

Signed-off-by: Dan Handley <dan.handley@arm.com>

.github/pull_request_template.md

@@ -1,8 +1,5 @@
 Notes:
-* Pull requests cannot be accepted until:
+* Pull requests cannot be accepted until the PR follows the [contributing guidelines](../CONTRIBUTING.md). In particular, each commit must have at least one `Signed-off-by:` line from the committer to certify that the contribution is made under the terms of the [Developer Certificate of Origin](../dco.txt).
--  The submitter has [accepted the online agreement here with a click through](https://developer.mbed.org/contributor_agreement/) 
-   or for companies or those that do not wish to create an mbed account, a slightly different agreement can be found [here](https://www.mbed.com/en/about-mbed/contributor-license-agreements/)
-- The PR follows the [mbed TLS coding standards](https://tls.mbed.org/kb/development/mbedtls-coding-standards)
 * This is just a template, so feel free to use/remove the unnecessary things
 ## Description
 A few sentences describing the overall goals of the pull request's commits.

CONTRIBUTING.md

@@ -5,11 +5,6 @@ We gratefully accept bug reports and contributions from the community. There are
  - As with any open source project, contributions will be reviewed by the project team and community and may need some modifications to be accepted.
  - The contribution should not break API or ABI, unless there is a real justification for that. If there is an API change, the contribution, if accepted, will be merged only when there will be a major release.
 
-Contributor License Agreement (CLA)
------------------------------------
-- All contributions, whether large or small, require a Contributor's License Agreement (CLA) to be accepted. This is because source code can possibly fall under copyright law and we need your consent to share in the ownership of the copyright.
-- To accept the Contributor’s License Agreement (CLA), individual contributors can do this by creating an Mbed account and [accepting the online agreement here with a click through](https://developer.mbed.org/contributor_agreement/). Alternatively, for contributions from corporations, or those that do not wish to create an Mbed account, a slightly different agreement can be found [here](https://www.mbed.com/en/about-mbed/contributor-license-agreements/). This agreement should be signed and returned to Arm as described in the instructions given.
-
 Coding Standards
 ----------------
 - We would ask that contributions conform to [our coding standards](https://tls.mbed.org/kb/development/mbedtls-coding-standards), and that contributions are fully tested before submission, as mentioned in the [Tests](#tests) and [Continuous Integration](#continuous-integration-tests) sections.
@@ -24,7 +19,8 @@ Making a Contribution
 1. Write a test which shows that the bug was fixed or that the feature works as expected.
 1. Send a pull request (PR) and work with us until it gets merged and published. Contributions may need some modifications, so a few rounds of review and fixing may be necessary. We will include your name in the ChangeLog :)
 1. For quick merging, the contribution should be short, and concentrated on a single feature or topic. The larger the contribution is, the longer it would take to review it and merge it.
-1. Mbed TLS is released under the Apache license, and as such, all the added files should include the Apache license header.
+1. All new files should include the [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) standard license header where possible.
+1. Ensure that each commit has at least one `Signed-off-by:` line from the committer. If anyone else contributes to the commit, they should also add their own `Signed-off-by:` line. By adding this line, contributor(s) certify that the contribution is made under the terms of the [Developer Certificate of Origin](dco.txt). The contribution licensing is described in the [License section of the README](README.md#License).
 
 API/ABI Compatibility
 ---------------------

README.md

@@ -167,21 +167,12 @@ Mbed TLS can be ported to many different architectures, OS's and platforms. Befo
 -   [What external dependencies does Mbed TLS rely on?](https://tls.mbed.org/kb/development/what-external-dependencies-does-mbedtls-rely-on)
 -   [How do I configure Mbed TLS](https://tls.mbed.org/kb/compiling-and-building/how-do-i-configure-mbedtls)
 
+License
+-------
+
+Unless specifically indicated otherwise in a file, Mbed TLS files are provided under the [Apache-2.0](https://spdx.org/licenses/Apache-2.0.html) license. See [here](./apache-2.0.txt) for the full text of this license. Contributors must accept that their contributions are made under both the Apache-2.0 AND [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) licenses. This enables LTS (Long Term Support) branches of the software to be provided under either the Apache-2.0 OR GPL-2.0-or-later licenses.
+
 Contributing
 ------------
 
-We gratefully accept bug reports and contributions from the community. There are some requirements we need to fulfill in order to be able to integrate contributions:
+We gratefully accept bug reports and contributions from the community. Please see the [contributing guidelines](CONTRIBUTING.md) for details on how to do this.
-
--   All contributions, whether large or small require a Contributor's License Agreement (CLA) to be accepted. This is because source code can possibly fall under copyright law and we need your consent to share in the ownership of the copyright.
--   We would ask that contributions conform to [our coding standards](https://tls.mbed.org/kb/development/mbedtls-coding-standards), and that contributions should be fully tested before submission.
--   As with any open source project, contributions will be reviewed by the project team and community and may need some modifications to be accepted.
-
-To accept the Contributor’s Licence Agreement (CLA), individual contributors can do this by creating an Mbed account and [accepting the online agreement here with a click through](https://os.mbed.com/contributor_agreement/). Alternatively, for contributions from corporations, or those that do not wish to create an Mbed account, a slightly different agreement can be found [here](https://www.mbed.com/en/about-mbed/contributor-license-agreements/). This agreement should be signed and returned to Arm as described in the instructions given.
-
-### Making a Contribution
-
-1.  [Check for open issues](https://github.com/ARMmbed/mbedtls/issues) or [start a discussion](https://forums.mbed.com/c/mbed-tls) around a feature idea or a bug.
-2.  Fork the [Mbed TLS repository on GitHub](https://github.com/ARMmbed/mbedtls) to start making your changes. As a general rule, you should use the "development" branch as a basis.
-3.  Write a test which shows that the bug was fixed or that the feature works as expected.
-4.  Send a pull request and bug us until it gets merged and published. Contributions may need some modifications, so work with us to get your change accepted. We will include your name in the ChangeLog :)
-

dco.txt

@@ -0,0 +1,37 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.