hooks: README for OpenStack Network vNIC hook

Added a readme file outlining the general behaviour of the hook.

Change-Id: I70586d2189baa6ec7c7fe2e4289aa72326fbdac2
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16002
Reviewed-by: Dan Kenigsberg <danken@redhat.com>