Add instruction to open a PostgreSQL shell session (#40)

README.md

@@ -85,23 +85,30 @@ web        ./manage.py runserver 0.0. ...   Up          0.0.0.0:8000->8000/tcp
 ```
 
 ### Run tests
+
 ```sh
 docker-compose exec -w /code/wagtail web python runtests.py
 ```
 
-### You can open a django shell session
+### Open a Django shell session
 
 ```sh
 docker-compose exec web python manage.py shell
 ```
 
-### You can open a shell on the web server
+### Open a PostgreSQL shell session
+
+```sh
+docker-compose exec web dbshell
+```
+
+### Open a shell on the web server
 
 ```sh
 docker-compose exec web bash
 ```
 
-### You can open a shell to work with the frontend code (Node/NPM)
+### Open a shell to work with the frontend code (Node/NPM)
 
 ```sh
 docker-compose exec frontend bash