adding linuxbridge agent feature to neutron formula.

by this user will be able to enable linuxbridge agent insted of default openvswitch from cluster level.

Pillar changes are :

neutron:gateway:backend:mechanism:lb:driver = linuxbridge
neutron:compute:backend:mechanism:lb:driver = linuxbridge
neutron:gateway:agents:l3:interface_driver = linuxbridge

Closes-Bug: https://mirantis.jira.com/browse/PROD-24715

Change-Id: Ic1ca59155a1833eea928450fca8172321f88602d
diff --git a/README.rst b/README.rst
index 00a74c4..56d3b38 100644
--- a/README.rst
+++ b/README.rst
@@ -819,6 +819,37 @@
             ovs:
               driver: openvswitch
 
+Neutron with LinuxBridge Agents
+-------------------------------
+
+.. code-block:: yaml
+
+    neutron:
+      server:
+        firewall_driver: iptables
+        backend:
+          mechanism:
+            lb:
+              driver: linuxbridge
+      ....
+      compute:
+        backend:
+          mechanism:
+            lb:
+              driver: linuxbridge
+      ....
+      gateway:
+        backend:
+          mechanism:
+            lb:
+              driver: linuxbridge
+        agents:
+          dhcp:
+            interface_driver: linuxbridge
+          l3:
+            interface_driver: linuxbridge
+
+
 Neutron with VLAN-aware-VMs
 ---------------------------