Allow setting some VXLAN paramters

Allow setting the multicast group and the VNI range that will be used
when using the ML2 plugin.
diff --git a/README.rst b/README.rst
index e8e633f..56f1b51 100644
--- a/README.rst
+++ b/README.rst
@@ -514,6 +514,22 @@
             use_ovs_ports:
             - float-to-ex
 
+Additonal VXLAN tenant network settings
+---------------------------------------
+
+The default multicast group of 224.0.0.1 only multicasts to a single subnet.
+Allow overriding it to allow larger underlay network topologies.
+
+Neutron Server
+
+.. code-block:: yaml
+
+    neutron:
+      server:
+        vxlan:
+          group: 239.0.0.0/8
+          vni_ranges: "2:65535"
+
 Neutron VLAN tenant networks with Network Nodes
 -----------------------------------------------