Documentation for Big Switch router rule extension

Adds documentation for the Big Switch Neutron
plugin router rules extension.

Implements: blueprint bigswitch-router-rules
Change-Id: I261e6e49efdcb069dd7f3b67377b08f54ae3922c
Fix last commit using "of" instead of "or"

It's SAN or NFS - not SAN of NFS, a recent commit
broke this.

Change-Id: Icf8d39173c4f70d0d7234a31c0ad10dffca929bc
Merge "Fix last commit using "of" instead of "or""
Merge "Documentation for Big Switch router rule extension"