support mistaken-pull-closer for automatically clearing incoming PRs

Change-Id: I2f69a8152958bc1425efe569bace1b6f9702896d
Reviewed-on: https://chromium-review.googlesource.com/c/1366918
Reviewed-by: Mark Mentovai <mark@chromium.org>

.github/mistaken-pull-closer.yml

@@ -0,0 +1,17 @@
+# The JSONPath filter expression used to identify which PRs to close.
+# The data filtered is the pull request data along with other metadata passed in
+# by probot.
+# See http://goessner.net/articles/JsonPath/
+# `true` will close all PRs.
+filters:
+  - true
+
+# The message to post to the closed PR.
+commentBody: |
+  Thanks for your contribution!  Unfortunately, we don't use GitHub pull
+  requests to manage code contributions to this repository.  Instead, please
+  see [README.md](../blob/master/README.md) which provides full instructions
+  on how to get involved.
+
+# Whether to add a label to the closed PR.
+addLabel: false