Add extensions list

This patch adds extensions list instead of hardcoded.

Change-Id: I369b6593557ccbc84f06ec0e3d06feabb7640257
Related-PROD: PROD-14689
diff --git a/README.rst b/README.rst
index e8ef14c..840fb2c 100644
--- a/README.rst
+++ b/README.rst
@@ -898,6 +898,19 @@
           Instance port in the stated subnet will be associated with the dynamically generated floating IP.
 
 
+
+Enable Neutron extensions (QoS, DNS, etc.)
+------------------------------------------
+.. code-block:: yaml
+
+    neutron:
+      server:
+        backend:
+          extension:
+            - dns
+            - qos
+
+
 Documentation and Bugs
 ======================