add networking write up for devstack

This explains the current state of networking in devstack, and a
couple of scenarios that people might want to try out for local
testing.

Change-Id: I2be35f4345bf9306c981ef6f0186b48da7d06772
diff --git a/doc/source/site-map.rst b/doc/source/site-map.rst
index 74b944b..801fc66 100644
--- a/doc/source/site-map.rst
+++ b/doc/source/site-map.rst
@@ -14,6 +14,7 @@
 
    overview
    configuration
+   networking
    plugins
    plugin-registry
    faq