Add gateway_mode configuration

Change-Id: I99ffab4ab4ffbfd251cf3604c59c52e6f5911d6c
diff --git a/README.rst b/README.rst
index 561e785..f858209 100644
--- a/README.rst
+++ b/README.rst
@@ -576,6 +576,19 @@
           dns: 127.0.0.1
           mtu: 9000
 
+
+Compute nodes with gateway_mode
+-------------------------------
+
+Gateway mode: can be server/ vcpe (default is none)
+
+.. code-block:: yaml
+
+    opencontrail:
+      compute:
+        gateway_mode: server
+
+
 Keystone v3
 -----------