Introduce CONTRIBUTING.md

[fwbrasil]

Topics I think are important to cover:

• How to build locally
• How to open a PR
• How to claim a bounty
• LLM use guide
• Code style guide
• Document the core contributors team and role

We could elaborate the sections with instructions to make these flows work well. For example, the LLM guide could help avoid low-effort automated PRs and I think we can have more strict guidance for bounties.