Add note on running tests locally

Author: kvz

README.md

@@ -42,3 +42,25 @@ For fully working examples, take a look at [`examples/`](https://github.com/tran
 ## Documentation
 
 See [readthedocs](https://transloadit.readthedocs.io) for full API documentation.
+
+## Contributing
+
+### Running tests
+
+If you have a global installation of `poetry`, you can run the tests with:
+
+```bash
+poetry run pytest --cov=transloadit tests
+```
+
+If you can't use a global installation of `poetry`, e.g. when using Nix Home Manager, you can create a Python virtual environment and install Poetry there:
+
+```bash
+python -m venv .venv && source .venv/bin/activate && pip install poetry && poetry install
+```
+
+Then to run the tests:
+
+```bash
+source .venv/bin/activate && poetry run pytest --cov=transloadit tests
+```