[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]

Test Results

    21 files      21 suites   3d 7h 43m 35s ⏱️
 3 863 tests  3 848 ✅   0 💤 15 ❌
72 810 runs  72 674 ✅ 121 💤 15 ❌

For more details on these failures, see this check.

Results for commit b66ca7f.