Improve our Contributing guide regarding squashing commits

[kinow]

One major issue blocking our release is the number of commits in the same PR, some not rebased.

Looking at the current CONTRIBUTING.md, I think it's not direct about number of commits in a pull request.

This issue is a follow-up so we remember to update it as soon as we can. IMHO, we must not simply say "squash everything into a single commit", but we must ask contributors to only not squash when it makes sense, but we can discuss that and see what others think about it too 👍