Add note to external ceph docs for pools/keyrings

When using external Ceph the operator must create pools for each service
and configure keyrings with appropriate permissions. The official Ceph
docs describe this in detail so let operators know this.

Change-Id: Ic3e52e1fbbf09ec09ac21b5b3067092b195812f1

doc/source/reference/storage/external-ceph-guide.rst

@@ -16,6 +16,9 @@ Requirements
 * Existing credentials in Ceph for OpenStack services to connect to Ceph
   (Glance, Cinder, Nova, Gnocchi, Manila)
 
+Refer to http://docs.ceph.com/docs/master/rbd/rbd-openstack/ for details on
+creating the pool and keyrings with appropriate permissions for each service.
+
 Enabling External Ceph
 ~~~~~~~~~~~~~~~~~~~~~~