Add line about localhost port number

So people don't have to look inside the compose file or run `docker ps
-a` to figure it out. (Also nice if they're not familiar with how
port-forwarding works in docker)

README.md

@@ -39,6 +39,8 @@ After setting up the development container, you can start it with
 docker compose up --detach
 ```
 
+The API should now be accessible through `localhost:8001`.
+
 And to stop BadgeHub
 
 ```bash