add doc section for ml2 extension drivers
add doc section for ml2 extension drivers

Today the ML2 config reference doesn't mention anything about ML2
extension drivers. While it likely doesn't make sense to clutter the
primary ML2 driver types (type/mech) with extension drivers, this patch
proposes we add a small section about them to link the concept to the
config reference.

Change-Id: Id964e6e4d13d400ea4cd8a1707ad0f08c52ef0b1
Partial-Bug: #1683102

mahak/neutron,huntxu/neutron,noironetworks/neutron,eayunstack/neutron,openstack/neutron,openstack/neutron,huntxu/neutron,mahak/neutron,openstack/neutron,mahak/neutron,eayunstack/neutron,noironetworks/neutron