docs(CONTRIBUTING): Mention commit message format

CONTRIBUTING.md

@@ -15,6 +15,11 @@ You generally only need to submit a CLA once, so if you've already submitted one
 (even if it was for a different project), you probably don't need to do it
 again.
 
+## Commit messages
+
+Commits in this repository follow the [Angular commit message
+guidelines][commits].
+
 ## Code reviews
 
 All submissions, including submissions by project members, require review. We
@@ -26,3 +31,5 @@ information on using pull requests.
 
 This project follows [Google's Open Source Community
 Guidelines](https://opensource.google.com/conduct/).
+
+[commits]: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit