docs: Update contribution policy

Author: skerkour

CONTRIBUTING.md

@@ -4,6 +4,8 @@ Contributions are welcome, especially bug reports, improvement ideas and new pac
 
 Except for minor typos, no pull request will be accepted without a preliminary discussion. Please open an issue first.
 
+Commits need to be signed and merged (squashed).
+
 ## AI Policy
 
 The use of AI-generated code is generally encouraged, **but**, AI-generated issues, text or any other form of communication will not be tolerated.