CLA should be mentioned in CONTRIBUTING file

[olivia-fl]

Describe the bug

Cinny has a CLA, stored in the cinnyapp/cla repo. This isn't mentioned in any of the docs. I only found out about it before submitting a patch by chance, because I saw the cla-assistant bot on somebody else's PR.

Projects that have CLAs but don't document them clearly tend to feel like a bit of a bait-and-switch, because you finish most of the work for your patch before finding out that it won't be accepted without a copyright assignment. This could be avoided by documenting the CLA in the CONTRIBUTING file.

Reproduction

No response

Expected behavior

No response

Platform and versions

NA

Additional context

No response