docs: add a `CONTRIBUTING.md` that references the dev docs

[agilgur5]

Summary

Add a standard CONTRIBUTING.md file to the root that just references docs/developer.md for better accessibility

Details

• CONTRIBUTING.md is a standard OSS convention

  • and GitHub recognizes when a CONTRIBUTING.md exists too and will add some buttons / prompts here and there
  • I looked for it initially when I started contributing and didn't see it, which was a bit confusing until I found docs/developer.md

• just point to docs/developer.md as that describes things already

  • this just serves so people can find the dev docs more easily