docs: add contributing guide to README.md

Add contributing guide to README.md based upon the suggestion of
@arjentz in #41.

Signed-off-by: Roald Nefs <info@roaldnefs.com>
Co-authored-by: Arjentz <4867268+arjentz@users.noreply.github.com>

README.md

@@ -90,6 +90,29 @@ class NaClIPAddressField(NaClFieldMixin, models.IPAddressField):
 
 Please report any issues you encounter when trying this.
 
+## Contributing
+
+Start a PostgreSQL Docker container for testing using:
+```bash
+docker run --rm -e POSTGRES_PASSWORD=postgres POSTGRES_DB=github_actions postgres
+```
+
+Then, you can run tests in another window using:
+```bash
+python -m venv env
+source env/bin/activate
+pip install '.[test]'
+python manage.py test
+```
+
+Before you contribute, make sure you lint your code:
+```bash
+pip install '.[lint]'
+flake8 --exclude .git,pycache,docs/conf.py,build,dist,env
+```
+
+After you code is done, free free to open a new pull request.
+
 ## References
 
 - <https://github.com/defrex/django-encrypted-fields>