docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+
+---
+
+# Contributing to Judge4C
+
+Thank you for your interest in contributing to **Judge4C**! We welcome all kinds of contributions, including bug reports, feature requests, code improvements, and documentation.
+
+## 📌 Branching Strategy
+
+* `main`: Stable release branch. **Do not** directly commit to this branch.
+* `develop`: Active development happens here. Please branch off from `develop` when working on new features or bug fixes.
+
+## 🧑‍💻 How to Contribute
+
+### 1. Fork the Repository
+
+Click the **Fork** button on the top right of the [GitHub repository](https://github.com/massbug/judge4c), then clone your fork locally:
+
+```bash
+git clone https://github.com/YOUR_USERNAME/judge4c.git
+cd judge4c
+```
+
+### 2. Create a New Branch
+
+Always branch off from `develop`:
+
+```bash
+git checkout develop
+git pull
+git checkout -b feature/your-feature-name
+```
+
+Use appropriate naming conventions:
+
+* `feature/xxx` for new features
+* `fix/xxx` for bug fixes
+* `docs/xxx` for documentation improvements
+
+### 3. Make Your Changes
+
+Follow the project's coding style and structure. Be concise and clear. Add comments where necessary.
+
+### 4. Commit Your Changes
+
+Use clear and descriptive commit messages:
+
+```bash
+git add .
+git commit -m "fix: correct issue with X"
+```
+
+Follow [Conventional Commits](https://www.conventionalcommits.org/) when possible.
+
+### 5. Push and Create a Pull Request
+
+```bash
+git push origin feature/your-feature-name
+```
+
+Then, go to the original repository and open a **Pull Request** (PR) to merge your changes into the `develop` branch.
+
+Please ensure your PR includes:
+
+* A clear description of your changes
+* Reference to any related issues (`Closes #issue-number`)
+* Test results (if applicable)
+
+### 6. Code Review and Merge
+
+Your PR will be reviewed by the maintainers. They may suggest changes or improvements. Once approved, your changes will be merged into `develop`, and later into `main` for the next release.
+
+## ✅ Contribution Checklist
+
+Before submitting a PR, please ensure:
+
+* [ ] Code compiles without errors
+* [ ] All tests pass (if applicable)
+* [ ] Code adheres to the style of the project
+* [ ] You've updated or added documentation as needed
+
+## 💬 Need Help?
+
+Open an [Issue](https://github.com/massbug/judge4c/issues) for general questions or discussions.
+
+---
+
+欢迎使用中文参与贡献讨论，但代码与提交信息请保持英文风格，以便国际开发者理解。