Updating the contribution guideline to request tests for all the code changes (#16)

Author: arsen-es

Diff for: CONTRIBUTING.md

@@ -31,7 +31,7 @@ To send us a pull request, please:
 
 1. Fork the repository.
 2. Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
-3. Ensure local tests pass.
+3. Ensure local tests pass; please add unit tests for all the new code paths introduced by your change.
 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.