Make reference to service-types-authority from plugins.rst
Make reference to service-types-authority from plugins.rst

We want people creating plugins (that add services) to be aware of the
service-types-authority (STA), so this change adds a Prerequisites
section and notes the existince of the STA there, and the need to apply
there to create a service-type.

Change-Id: I1aa48fe231aaa4499f8b4fe336abea668841b9af

openstack-dev/devstack,noironetworks/devstack,dtroyer/devstack,nawawi/openstack,group-policy/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,group-policy/devstack