Add note about being in the `qa` directory for `bin/qa` to work

I ran into some issues when trying it out
https://gitlab.slack.com/archives/C3JJET4Q6/p1517384135000134

qa/README.md

@@ -34,6 +34,9 @@ You can use GitLab QA to exercise tests on any live instance! For example, the
 following call would login to a local [GDK] instance and run all specs in
 `qa/specs/features`:
 
+First, `cd` into the `$gdk/gitlab/qa` directory.
+The `bin/qa` script expects you to be in the `qa` folder of the app.
+
 ```
 bin/qa Test::Instance http://localhost:3000
 ```