doc: readme update

- Explicitly state that in order to use SSO with existing LMS admin users, the newly created superuser in discovery must have the same username and email as the LMS user.

README.rst

@@ -48,9 +48,13 @@ commands from the UI, a user must be created:
 
 Then, you must log in with this user at http://discovery.local.openedx.io/admin.
 
-Alternatively, you can log in with oauth2 using a pre-existing user created on the LMS/CMS by accessing
-http(s)://discovery.<your lms host>/login. To do so, the proper domain names must exist and point to
-the production server.
+Using SSO with LMS
+~~~~~~~~~~~~~~~~~~
+
+If you want to log in using Single Sign-On (SSO) with the LMS, ensure that the superuser you created 
+above in discovery has the same username and email as an existing admin user in the LMS/CMS. You can 
+then access the discovery interface via `http(s)://discovery.<your lms host>/login`. Make sure that 
+the proper domain names are configured and point to the production server.
 
 Index configuration
 ~~~~~~~~~~~~~~~~~~~