Add go generate mention to CONTRIBUTING.md

CONTRIBUTING.md

@@ -64,6 +64,10 @@ are more sensitive, emailed to <opensource@google.com>.
      messages are able to be formatted properly by various git tools.
 
   1. Finally, push the commits to your fork and submit a [pull request][].
+     Before pushing commits, it is highly advised to check for generated files
+     that were either created or modified for the sake of your commit. Running
+     `go generate -x ./...` should return a log of modified generated files that should
+     be included alongside the manually written code in the commit.
      **NOTE:** Please do not use force-push on PRs in this repo, as it makes
      it more difficult for reviewers to see what has changed since the last
      code review.