Tweak Contributing docs to help people run the tests

docs/contributing.rst

@@ -54,11 +54,17 @@ Then start the ``compose`` session::
 
 At this point, you should be able to see the Web UI at http://localhost:8000
 
-Once your session is up and running, you can make yourself a superuser account::
+Once your session is up and running, you can:
+
+…make yourself a superuser account::
 
     docker compose -f docker/docker-compose.yml exec web python3 manage.py createsuperuser
 
-And you can run the tests inside your container::
+…install the test dependencies inside your container::
+
+    docker compose -f docker/docker-compose.yml exec web pip install -r requirements-dev.txt
+
+…run the tests inside your container::
 
     docker compose -f docker/docker-compose.yml exec web pytest