docs: external fork contirbutors instructions

[jcucuzza]

🎯 What does this PR do?

Instruct all earthlings (potentially Martians in the future 👽 ) how to contribute to our chip 🍪.

✅ Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation (if needed).
• My commit messages follow the Conventional Commits standard.