Add documentation on the usage of keystone's scope & roles

As Tempest support the keystone's scope and new default roles
like reader, this commit document that how those can be
requested and used in Tempest or its plugins tests.

Change-Id: Iebacbeda231f82d6d16dbdcde635f19ae862181f
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index c43e420..c790c5f 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -153,6 +153,11 @@
 
 Pre-Provisioned Credentials are also known as accounts.yaml or accounts file.
 
+Keystone Scopes & Roles Support in Tempest
+""""""""""""""""""""""""""""""""""""""""""
+For details on scope and roles support in Tempest,
+please refer to :doc:`this document <keystone_scopes_and_roles_support>`
+
 Compute
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