Document port name query

[gianluca-sabena]

If you have a service with multiple ports it is possible to query a specific port. This allow to create a dns service discovery.

Document this feature here http://mesosphere.github.io/mesos-dns/docs/naming.html

marathon.json

...
     "portMappings": [
       {
         "containerPort": 9100,
         "hostPort": 0,
         "protocol": "tcp",
         "name": "http-metrics"
       },
       {
         "containerPort": 9000,
         "hostPort": 0,
         "protocol": "tcp",
         "name": "http-api"
       }
     ],
...

Query is like dig @mesos-dns _http-api._<task-name>._tcp.marathon.mesos