[doc] add CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Contributing to Xbyak
+
+Thank you for considering contributing to the Xbyak project. This document provides guidelines on how to contribute.
+
+## Bug Reports and Feedback
+
+If you find a bug, have a feature request, or have questions, please open an issue. Include the following information:
+
+- Detailed description of the problem
+- Steps to reproduce
+- Expected behavior
+- Actual behavior
+- Environment details (OS, compiler version, etc.)
+
+## Creating Pull Requests
+
+If you want to add features or make fixes, follow these steps to create a pull request:
+
+1. Fork the repository
+2. Create a new branch: `git checkout -b my-feature-branch`
+3. Make your changes
+4. Run tests and ensure all tests pass
+5. Commit your changes: `git commit -am 'Add new feature'`
+6. Push the branch: `git push origin my-feature-branch`
+7. Create a pull request
+
+When creating a pull request, clearly describe the changes and include any related issue numbers.
+
+## License
+
+Xbyak is released under the BSD-3-Clause License. Any code contributions will be licensed under the same license.
+