Add instruction to clean up commits before PRs are merged

Author: C0urante

CONTRIBUTING.md

@@ -35,6 +35,7 @@ To send us a pull request, please:
 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.
+7. Before merging, clean up the commit history for the PR. Each commit should be self-contained with an informative message, since each commit will be added to the history for this project.
 
 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/).