Add instructions on how to update the repo to the readme

Readme.md

@@ -2,3 +2,22 @@ Boost libraries - trimmed down for Citra
 ========================================
 
 This is a subset of Boost v1.63.0 generated using the bcp tool. To get a list of boost modules guaranteed to exist, check the build script.
+
+Updating this repo (on Windows)
+===============================
+
+To update the Boost version (or to add a new library) follow these steps:
+
+  - Download Boost and extract the package, then launch Powershell and `cd` to the `boost_1_xx_0` directory.
+  - Build the `bcp` tool:
+    ```
+    .\boostrap.bat
+    .\b2 tools\bcp
+    ```
+
+  - Store the boost directory in a variable for later use: `$boost_dir = $pwd`.
+  - Add bcp to your path: `$env:Path += $boost_dir\bin.v2\tools\bcp\msvc-14.1\release\link-static\threading-multi` (The correct output path should be printed by b2 during the build.)
+  - `cd` to this repo's directory (`...\externals\boost\`)
+  - Remove the existing boost from the repo: `rm -r boost` (This is only necessary if doing a Boost version upgrade, in case they removed any files in the new version.)
+  - Run `.\build.cmd $boost_dir` to build a new trimmed down distro.
+  - Add/remove all files in git and commit.