Update README.rd, Add missing information on ADDITONAL_SERVICES for Swarm Setups

There are limitations on using links for identification of load balances services when using Swarm with overlay network.
Swarm will still try to schedule linked services together on same node.

There is already a second option which is to pass in load balanced service names via the ADDITIONAL_SERVICES envvar instead of using links but was not really documented. This removes some of the issues legacy links has when using Swarm.
I was gonna add a non-link dependent option but someone already did the work ;)