add commit message guidance #336

Since many Google projects refer to this site for writing git commit
messages already, or have guidance that is well aligned with it, link
to them rather than write our own document.

Fixes: #336

README.md

@@ -53,6 +53,12 @@ The following Google style guide lives outside of this project:
 *  [Effective Dart][dart]
 *  [Kotlin Style Guide][kotlin]
 
+Since projects are largely maintained in a [VCS], writing good commit messages
+is important to long term project health. Please refer to [How to Write a Git
+Commit Message](https://cbea.ms/git-commit/) as an excellent resource. While it
+explicitly refers to the Git [SCM], its principles apply to any system, and many
+Git conventions are trivial to translate to others.
+
 ## Contributing
 
 With few exceptions, these style guides are copies of Google's internal style
@@ -92,3 +98,5 @@ primarily optimizing for Google's internal needs.
 [xml]: https://google.github.io/styleguide/xmlstyle.html
 [dart]: https://www.dartlang.org/guides/language/effective-dart
 [ccl]: https://creativecommons.org/licenses/by/3.0/
+[SCM]: https://en.wikipedia.org/wiki/Source_control_management
+[VCS]: https://en.wikipedia.org/wiki/Version_control_system