Update add_gcm.rb with a comment in case of an upgrade

Just a small explanation: the migration might fail if the index_rapns_notifications_multi doesn't exists and it's actually called index_rapns_notifications_on_delivered_failed_deliver_after.

This depends on the base version of rapns from where you're upgrading from. I for example upgraded from version 2.0.5, revision 0f4b90dde002d503b4eb6c2187c954180799a349 and it seems the index was renamed later.