Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to decimal-bytes
+
+Thanks for your interest in contributing to decimal-bytes!
+
+## Prerequisites
+
+- **Rust 1.85+** (the minimum supported Rust version)
+
+## Development
+
+```bash
+cargo build                          # Default build
+cargo build --features rust_decimal  # With rust_decimal interop
+cargo build --features bigdecimal    # With bigdecimal interop
+cargo test --all-features            # Run all tests
+cargo bench                          # Run benchmarks
+```
+
+Before submitting a PR, make sure CI checks pass locally:
+
+```bash
+cargo fmt --all -- --check
+cargo clippy --all-features -- -D warnings
+cargo test --all-features
+```
+
+## Submitting Changes
+
+1. Fork the repository and create a branch from `main`.
+2. Add tests for any new functionality or bug fixes.
+3. Run the checks above.
+4. Open a pull request against `main`.
+
+## Reporting Issues
+
+Open an issue on [GitHub](https://github.com/paradedb/decimal-bytes/issues) with a clear description of the problem or feature request.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).