docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to c32check
+
+Thank you for your interest in contributing to c32check! This library provides Crockford base-32 encoding with checksums for the Stacks blockchain.
+
+## How to Contribute
+
+### Reporting Issues
+
+- Check existing issues before creating a new one
+- Provide a clear description of the problem
+- Include steps to reproduce the issue
+- Specify the version of c32check you're using
+
+### Submitting Pull Requests
+
+1. **Fork the repository** and create your branch from `main`
+2. **Install dependencies**: `npm install`
+3. **Make your changes** following the coding style of the project
+4. **Write tests** for any new functionality
+5. **Run the test suite**: `npm test`
+6. **Ensure linting passes**: `npm run lint`
+7. **Commit your changes** with clear, descriptive commit messages
+8. **Push to your fork** and submit a pull request
+
+### Development Setup
+
+```bash
+# Clone your fork
+git clone https://github.com/YOUR_USERNAME/c32check.git
+cd c32check
+
+# Install dependencies
+npm install
+
+# Run tests
+npm test
+
+# Build the project
+npm run build
+```
+
+### Code Style
+
+- Follow the existing TypeScript coding patterns in the project
+- Use meaningful variable and function names
+- Add JSDoc comments for public functions
+- Keep functions focused and single-purpose
+
+### Testing
+
+- Write unit tests for new features
+- Ensure all existing tests pass before submitting
+- Test edge cases, especially for encoding/decoding functions
+- Run `npm run test:big-data` for extensive test coverage
+
+### Pull Request Guidelines
+
+- Keep PRs focused on a single feature or fix
+- Reference any related issues in the PR description
+- Update documentation if your changes affect the public API
+- Be responsive to feedback and code review comments
+
+## Questions?
+
+If you have questions about contributing, feel free to open an issue for discussion.
+
+## License
+
+By contributing to c32check, you agree that your contributions will be licensed under the MIT License.