added CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing to This Tic-Tac-Toe Project
+
+Thank you for your interest in contributing!  
+This is a small and lightweight Tic-Tac-Toe game, so the contribution process is intentionally simple.
+
+---
+
+## How to Contribute
+
+### 1. Fork the repository
+Create your own fork and work on your changes in a separate branch.
+
+### 2. Keep the code simple
+The project aims to stay small, readable, and easy to understand.  
+Avoid unnecessary complexity or large framework additions.
+
+### 3. Screen-clear requirement (important!)
+This project clears the screen between turns.  
+To keep the code minimal, we do **not** implement OS-specific detection.
+
+If you are on **Linux or Unix**, please add this alias to your shell:
+
+`$ alias cls="clear"`
+
+This ensures the game behaves consistently across systems without adding platform-specific code.
+
+### 4. Test your changes
+Before submitting a pull request, make sure the game still runs cleanly and behaves as expected.
+
+### 5. Submit a Pull Request
+Please describe:
+- what you changed
+- why you changed it
+- anything reviewers should know
+
+Small, focused pull requests are preferred.
+
+---
+
+## Code Style
+
+- Keep functions short and readable
+- Use clear variable names
+- Prefer simplicity over cleverness
+
+---
+
+## Reporting Issues
+
+If you find a bug or have a suggestion, feel free to open an issue.  
+Please include steps to reproduce problems when possible.
+
+---
+
+Thank you for helping improve this project!