Document local test setup and pytest commands

.github/CONTRIBUTING.md

@@ -1,5 +1,37 @@
 # Contributing
 
+## Running tests
+
+Use Python 3.10 or newer. From the repository root, create a virtual environment:
+
+```sh
+python -m venv .venv
+```
+
+Activate it with `source .venv/bin/activate` on Linux or macOS, or
+`.venv\Scripts\Activate.ps1` in Windows PowerShell. Then install the project and its
+test dependencies:
+
+```sh
+python -m pip install -e ".[tests]"
+```
+
+Run the full suite, a single file, or tests matching a name:
+
+```sh
+python -m pytest
+python -m pytest tests/test_number.py
+python -m pytest tests/test_number.py -k intcomma
+```
+
+Some localisation tests are skipped when compiled `.mo` translation files are missing.
+Generate them with `scripts/generate-translation-binaries.sh`, which requires Bash and
+GNU gettext's `msgfmt`. Translation-update tests also require `xgettext` and `msgmerge`.
+Use `python -m pytest -rs` to see skip reasons.
+
+CI generates translation binaries and runs tests through [tox](../tox.ini). See the
+[test workflow](workflows/test.yml) for supported Python versions and platforms.
+
 ## Linting
 
 Linting is run on the CI using [prek](https://prek.j178.dev//), and can be run locally: