update testing documentation

Author: mishaschwartz

README.md

@@ -96,6 +96,16 @@ Then the `/test` route will not be available from `/v3` onwards.
 To run tests:
 
 ```sh
-python3 -m pip install -r requirements.test.txt
-pytest test/
+python3 -m pip install .[dev]
+MONGODB_URI="mongodb://localhost:27017" pytest ./test
+```
+
+This assumes that you have a mongodb service running at `mongodb://localhost:27017`.
+
+Alternatively you can run start up the development stack with docker compose and then
+run tests in the docker container:
+
+```sh
+docker compose -f docker-compose.dev.yml up -d
+docker compose exec marble_api pytest ./test
 ```