Improved README.rst
Improved README.rst

Improve README.rst, in particular to
properly distinguish the use for Neutron ML2 and
for networking-bgpvpn.

Change-Id: I0978a07910232906bc928413f8554f4e6486e099

openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2