[nfc] update CONTRIBUTING.md to add basic LLM policy

[silverweed]

This Pull request:

Adds a section about LLM usage to CONTRIBUTING.md. It is not meant as a fully-fledged "AI policy" but just some basic common-sense guidelines that are nonetheless good to spell out.

Also adds a section about branch names, since opening a PR with a branch named master conflicts with our CI.

[ferdymercury]

Thanks!
Maybe also mention that copy pasting could lead to copyright infringement eg if LLM is copying verbatim from a GPL project into LGPL

[github-actions]

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[guitargeek]

Very nice, I think this is well-written and captures the essential etiquette!

Going further, we might spell out precisely what it means "to disclose whether the proposed changes (code or otherwise) were automatically generated", maybe with templates for such disclosures that can be added to commit messages and PR descriptions.

[dpiparo]

Thanks for this initiative. Before merging, even if I do not see huge hurdles ahead, I was thinking to discuss this a bit broadly within the core developers.