Add README for api-ref
Add README for api-ref

In the neutron stadium, we maintain the networking API reference
in neutron-lib repository. It is different from other projects,
so it would be helpful for contributors if we have README in the
usual location (neutron/api-ref).

Depends-On: I6f2c4d476c31b252bfb181138505673f397639fc
Change-Id: I41dc58e5edfd18a52f13f042ba865b82fad764f7

mahak/neutron,igor-toga/local-snat,mahak/neutron,cloudbase/neutron,mahak/neutron,noironetworks/neutron,cloudbase/neutron,huntxu/neutron,sebrandon1/neutron,openstack/neutron,eayunstack/neutron,openstack/neutron,eayunstack/neutron,noironetworks/neutron,sebrandon1/neutron,igor-toga/local-snat,huntxu/neutron,openstack/neutron