Add support for setting extra networking configuration options.

Add support for generically setting arguments for OpenStack Networking
for both the server and agent. This is done using Q_SRV_EXTRA_OPTS for the
server, which let you place settings in the "OVS" or "LINUX_BRIDGE"
sections of the plugin configuration file. For the agent, two variables
are defined: Q_AGENT_EXTRA_AGENT_OPTS lets you set items in the "AGENT"
section of the plugin configuration file, and Q_AGENT_EXTRA_SRV_OPTS
lets you set things in the "OVS" or "LINUX_BRIDGE" sections of the
plugin configuration file.

Implements blueprint devstack-quantum-config.

Change-Id: I9ba44b21231f88cda1bcc5e3389c7875f03d8145
diff --git a/README.md b/README.md
index 8573638..905a54d 100644
--- a/README.md
+++ b/README.md
@@ -122,6 +122,19 @@
 
 Then run `stack.sh` as normal.
 
+devstack supports adding specific Quantum configuration flags to both the Open vSwitch and LinuxBridge plugin configuration files. To make use of this feature, the following variables are defined and can be configured in your `localrc` file:
+
+    Variable Name             Plugin Config File Section Modified
+    -------------------------------------------------------------------------------------
+    Q_SRV_EXTRA_OPTS          `OVS` (for Open Vswitch) or `LINUX_BRIDGE` (for LinuxBridge)
+    Q_AGENT_EXTRA_AGENT_OPTS  AGENT
+    Q_AGENT_EXTRA_SRV_OPTS    `OVS` (for Open Vswitch) or `LINUX_BRIDGE` (for LinuxBridge)
+
+An example of using the variables in your `localrc` is below:
+
+    Q_AGENT_EXTRA_AGENT_OPTS=(tunnel_type=vxlan vxlan_udp_port=8472)
+    Q_SRV_EXTRA_OPTS=(tenant_network_type=vxlan)
+
 # Tempest
 
 If tempest has been successfully configured, a basic set of smoke tests can be run as follows: