Add CONTRIBUTING.md

Author: homebysix

CONTRIBUTING.md

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+# Contributing
+
+Thanks for considering a contribution!
+
+## Setup
+
+```sh
+python3 -m venv .venv
+.venv/bin/pip install -e .
+.venv/bin/pip install pre-commit
+.venv/bin/pre-commit install
+```
+
+## Running tests
+
+```sh
+.venv/bin/python -m unittest discover -vs tests
+```
+
+Tests also run in CI on Python 3.10–3.14. New hooks should ship with tests
+in `tests/`.
+
+## Linting and formatting
+
+`pre-commit` runs `black`, `isort`, `flake8`, and `pyupgrade`. Either let
+the installed hook handle it on commit, or run it manually:
+
+```sh
+.venv/bin/pre-commit run --all-files
+```
+
+pre-commit.ci also runs these hooks on every PR.
+
+## Adding a new hook
+
+A new hook needs:
+
+- An entry in `.pre-commit-hooks.yaml`
+- An entry point in `setup.py`
+- A usage example in `README.md`
+- Tests in `tests/`