Doc change: Add missing relation for Keystone OIDC configuration

This change adds documentation on another relation you must have
in order for you to configure Keystone and Horizon or logins
using OpenIDC.

Partial-Bug: 2065590
Change-Id: I0deba42ef4869d5f10229ebe1aac08b8e68d3009

Author: jadonn

README.md

@@ -34,6 +34,11 @@ Join keystone-openidc to openstack-dashboard to provide SSO access through Horiz
 
     juju add-relation openstack-dashboard:websso-fid-service-provider keystone-openidc:websso-fid-service-provider
 
+Enable Horizon as a trusted dashboard for Web Single Single-On for Keystone:
+
+    juju add-relation openstack-dashboard:websso-trusted-dashboard keystone:websso-trusted-dashboard
+
+**You must add this relation for Horizon and Keystone. If you do not, Keystone will return a 401 error that the login domain for Horizon is not a trusted domain.**
 
 Now provide an OpenID Connect client credentials and the URL for autodiscovery
 of the backend's configuration: