Add command to README for retrieving admin password

The admin password is no longer hardcoded so update the README
with details for how to retrieve the password.

Change-Id: I72e79a6abce089d0da8e9bb1d27f120fbd5cc49f

README.md

@@ -66,11 +66,10 @@ image, the user account is 'cirros'.
 
 ## Horizon
 
-The launch output will also provide information for the Horizon dashboard. Its
-credentials are:
+The launch output will also provide information for the Horizon dashboard. The
+username is 'admin' and the password can be obtained in this way:
 
-    username: admin
-    password: keystone
+    sudo snap get microstack config.credentials.keystone-password
 
 ## Removing MicroStack