Add support for n-g-s ml2 plugin

This patch allows to configure networking-genric-switch ml2 neutron
plugin by using the following pillar:

  neutron:
    server:
      backend:
        mechanism:
          ngs:
            driver: genericswitch
      n_g_s:
        enabled: true
        coordination:
          enabled: true
          backend_url: "http:/asdassadas"
        devices:
          s1brbm:
            options:
              device_type:
                value: netmiko_ovs_linux
              ip:
                value: 198.72.124.184
              username:
                value: ngs_ovs_manager
              password:
                value: password

Related-Prod: PROD-25542 (PROD-25542)

Change-Id: Ie9a23ad4b874a3b6eb2fb3a4e0e41b2018611da6
diff --git a/README.rst b/README.rst
index 86afc25..2812d40 100644
--- a/README.rst
+++ b/README.rst
@@ -535,6 +535,34 @@
             ovs:
               driver: openvswitch
 
+Configuring networking-generic-switch ml2 plugin used for
+baremetal integration:
+
+.. code-block:: yaml
+
+    neutron:
+      server:
+        backend:
+          mechanism:
+            ngs:
+              driver: genericswitch
+        n_g_s:
+          enabled: true
+          coordination:
+            enabled: true
+            backend_url: "etcd3+http://1.2.3.4:2379"
+          devices:
+            s1brbm:
+              options:
+                device_type:
+                  value: netmiko_ovs_linux
+                ip:
+                  value: 1.2.3.4
+                username:
+                  value: ngs_ovs_manager
+                password:
+                  value: password
+
 Network Node:
 
 .. code-block:: yaml