Setting up driver aliases for keystone domains

Issue: Starting from pike, keystone fails to initialize drivers
       by full path. Error from the log:
       ImportError: (u'Unable to find %(name)r driver in
       %(namespace)r.', {'namespace': 'keystone.identity', 'name':
       'keystone.identity.backends.ldap.Identity'})

Fix:   Use driver aliases taken from keystone egg entry_points.txt

Change-Id: I1dbf916741bd84d4a5e8963d0bfced2b786ac512
diff --git a/README.rst b/README.rst
index 6e4d1cc..aad2c64 100644
--- a/README.rst
+++ b/README.rst
@@ -214,6 +214,26 @@
               uid: keystone
               password: password
 
+Use driver aliases for drivers instead of class path's
+
+.. code-block:: yaml
+
+    keystone:
+      server:
+        domain:
+          test:
+            description: "Test domain"
+            backend: ldap
+            assignment:
+              backend: sql
+              driver: sql
+            identity:
+              backend: ldap
+              driver: keystone.identity.backends.ldap.Identity
+            ldap:
+              url: "ldaps://idm.domain.com"
+              ...
+
 Using LDAP backend for default domain
 
 .. code-block:: yaml