feat: Add root CONTRIBUTING.md to redirect users to official guide (Closes #11986)

CONTRIBUTING.md

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+# 📚 How to Contribute to Free Programming Books
+
+**Welcome!** We are thrilled you want to help expand and maintain this list. Due to the high volume of contributions, we have strict, detailed guidelines.
+
+**Your contribution will only be accepted if it adheres to these rules.**
+
+## 🔍 Official Contribution Guide
+
+Please read the full, official guidelines before submitting any Pull Request:
+
+👉 **[READ THE FULL CONTRIBUTING GUIDE HERE](https://ebookfoundation.github.io/free-programming-books/docs/CONTRIBUTING.html)**
+
+***
+
+## 💡 Quick Checklist for PRs
+
+Before submitting your Pull Request, please ensure:
+
+* The resource is genuinely **100% free** without requiring a working email address or mandatory registration.
+* The link is the **most authoritative source** (e.g., the author's official website).
+* The link uses **HTTPS** when available.
+* Your new entry is **alphabetized** correctly within its section.
+* You **do not** use file hosting services (Google Drive, Dropbox, etc.).
+
+If you have general questions about the workflow, please read the **[How-To Guide](https://ebookfoundation.github.io/free-programming-books/docs/HOWTO.html)**.
+
+Thank you for helping us maintain the world's largest list of free programming resources!