Update CONTRIBUTING to reflect how CI works in our repos (#369)

CONTRIBUTING.md

@@ -34,7 +34,8 @@ To send us a pull request, please:
 3. Ensure local tests pass.
 4. Commit to your fork using clear commit messages.
 5. Send us a pull request, answering any default questions in the pull request interface.
-6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.
+6. Wait for a repository collaborator to look at your PR, run it in CI, test, and review. If additional changes or discussion is needed, a contributor will get back to you, so please stay invovled in the conversation.
+    * Note: PRs from forks will not run in CI automatically for security reasons. If you make a PR and see that CI is left pending, this is normal and expected.
 
 GitHub provides additional document on [forking a repository](https://help.github.com/articles/fork-a-repo/) and
 [creating a pull request](https://help.github.com/articles/creating-a-pull-request/).