Add a CONTRIBUTING.md file.

Change-Id: I4e1ed0aaddf4dc516a81155ef62dba138f8495ae
Reviewed-on: https://boringssl-review.googlesource.com/7120
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/README.md b/README.md
index d8a65c2..e306da9 100644
--- a/README.md
+++ b/README.md
@@ -28,3 +28,4 @@
   * [STYLE.md](/STYLE.md): rules and guidelines for coding style.
   * include/openssl: public headers with API documentation in comments. Also [available online](https://commondatastorage.googleapis.com/chromium-boringssl-docs/headers.html).
   * [FUZZING.md](/FUZZING.md): information about fuzzing BoringSSL.
+  * [CONTRIBUTING.md](/CONTRIBUTING.md): how to contribute to BoringSSL.