fix(docs): Add note about commit messages (#155); r=rfk

CONTRIBUTING.md

@@ -21,6 +21,8 @@ Patches should be submitted as pull requests. When submitting patches as PRs:
 - Add both your code and new tests if relevant.
 - Run `npm test` to make sure all tests still pass.
 - Please do not include merge commits in pull requests; include only commits with the new relevant code.
+- Your commit message must follow the
+[commit guidelines](https://github.com/mozilla/fxa/blob/master/CONTRIBUTING.md#git-commit-guidelines).
 
 See the main [README.md](/README.md) for information on prerequisites, installing, running and testing.