Update readme with info on how to run tests with correct working directory

README.md

@@ -71,7 +71,7 @@ web        ./manage.py runserver 0.0. ...   Up          0.0.0.0:8000->8000/tcp
 
 ### Run tests
 ```sh
-docker-compose exec web python ../wagtail/runtests.py
+docker-compose exec -w /code/wagtail web python runtests.py
 ```
 
 ### You can open a django shell session