add url to prom docs

docs/prometheus.md

@@ -1,11 +1,19 @@
-# Prometheus Support
+# Prometheus Container
 
 ## Development
 AWX comes with an example prometheus container and make target.  To use it:
 
 1.  Edit `tools/prometheus/prometheus.yml` and update the `basic_auth` section
     to specify a valid user/password for an AWX user you've created.
     Alternatively, you can provide an OAuth2 token (which can be generated at
-    `/api/v2/users/N/personal_tokens/`).
+    `/api/v2/users/N/personal_tokens/`).  
+
+    > Note: By default, the config assumes a user with username=admin and password=password.
+    
 2.  Start the Prometheus container:
     `make prometheus`
+3.  The Prometheus UI will now be accessible at `http://localhost:9090/graph`.
+
+There should be no extra setup needed.  You can try executing this query in the 
+UI to get back the number of active sessions: `awx_sessions_total`
+