docs: add concise CONTRIBUTING.md following repo style

[jidris-spec]

Summary

I added a short CONTRIBUTING.md file to make it easier for new contributors to understand how to get started.

What’s inside

• Simple steps for forking, cloning, creating a branch, and submitting a pull request.
• A quick note about using clear commit messages and following the repo’s style.
• Small reminders about labeling issues like “good first issue” and documentation updates.

Why this change

Many open-source beginners rely on contribution guidelines to learn how to help.
This file gives them an easy starting point and keeps contributions consistent.

Checklist

• Added CONTRIBUTING.md
• Reviewed for clarity and style
• Matches the repo’s tone