Clarify documentation around plugins and additions archives
Clarify documentation around plugins and additions archives

Change-Id: I339874bcdb14f46b211bbab7a915d27310bb862d

rahulunair/kolla,stackforge/kolla,intel-onp/kolla,openstack/kolla,nihilifer/kolla,GalenMa/kolla,coolsvap/kolla,rahulunair/kolla,intel-onp/kolla,GalenMa/kolla,coolsvap/kolla,stackforge/kolla,nihilifer/kolla,stackforge/kolla,mrangana/kolla,coolsvap/kolla,mrangana/kolla,openstack/kolla