chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Contributions are welcome! Whether you're fixing a bug, improving documentation, or adding new features — thank you for taking the time to contribute.
+
+## Table of Contents
+
+- [Code of Conduct](#code-of-conduct)
+- [Opening Issues](#opening-issues)
+- [Pull Requests](#pull-requests)
+- [Local Setup](#local-setup)
+- [Running Tests](#running-tests)
+- [Running Static Analysis](#running-static-analysis)
+- [Deployment and Publishing](#deployment-and-publishing)
+
+## Code of Conduct
+
+Please be respectful and considerate in all interactions. Constructive feedback is welcome; harassment or hostile behavior is not.
+
+## Opening Issues
+
+We appreciate bug reports, feature requests, and general feedback.
+
+- **Bug reports**: Please include a minimal reproducible example and the Python version you're using.
+- **Feature requests or larger changes**: Please open an issue **before** starting implementation work. This allows us to discuss the scope and approach, and avoids wasted effort if the direction needs to change.
+- **Questions and discussions**: Issues are a good place for these too.
+
+## Pull Requests
+
+Pull requests are welcome! Please keep the following in mind:
+
+- **Scope**: Each pull request should be focused on **one thing only** — one bug fix, one feature, one refactor. Mixed-purpose PRs are harder to review and will likely be asked to be split up.
+- **Discuss first**: For any larger refactoring or significant code changes, please open an issue to discuss the matter before submitting a PR.
+- **Tests**: Please ensure your changes are covered by tests. Existing tests must continue to pass.
+- **Style**: Follow the conventions already established in the codebase.
+
+## Local Setup
+
+**Prerequisites**: Python 3.9 or newer.
+
+1. Clone the repository:
+
+   ```sh
+   git clone https://github.com/jkowalleck/rfc3987-syntax2.git
+   cd rfc3987-syntax2
+   ```
+
+2. Install the package in editable mode together with the development dependencies:
+
+   ```sh
+   pip install -e ".[testing]"
+   ```
+
+## Running Tests
+
+Tests are written with [pytest](https://docs.pytest.org/).
+
+```sh
+python -m pytest tests
+```
+
+## Running Static Analysis
+
+Type checking is done with [mypy](https://mypy.readthedocs.io/).
+
+```sh
+python -m mypy -v
+```
+
+The mypy configuration is in [`.mypy.ini`](.mypy.ini).
+
+## Deployment and Publishing
+
+Releases are managed by the maintainers. The process is:
+
+1. A maintainer creates a **GitHub Release** (including a version tag).
+2. This triggers the [`python-publish`](.github/workflows/python-publish.yml) GitHub Actions workflow, which builds the package and publishes it to [PyPI](https://pypi.org/project/rfc3987-syntax2/) automatically.
+
+Contributors do not need to do anything special for releases.