Add CONTRIBUTING.md

[Jolah1]

The repository currently lacks a CONTRIBUTING.md file, which can make onboarding new contributors more difficult.

This could cover:

• Dev environment setup
• How to build and run the project
• Running tests
• Code style & architecture expectations
• How to open issues and submit PRs
• Security considerations

[tnull]

Thanks, noted. I agree we should finally sit down and work something out. Maybe we want to work out a common policy across projects (also seems LDK's CONTRIBUTING.md could benefit from a refresher/rewrite?).

Thoughts, @TheBlueMatt @benthecarman ?

[benthecarman]

I have a minimal one in ldk-server (https://github.com/lightningdevkit/ldk-server/blob/main/CONTRIBUTING.md) but its more how-to stuff rather than actual guidelines. I agree would be good to have a unified policy, especially now with AI likely needs to be revisited