Add initial resources.md for discussion

The first draft of a Kubernetes resource model.
Marathon (a popular Mesos application) makes use of the ports resource.  See [TaskBuilder.getPorts](https://github.com/mesosphere/marathon/blob/master/src/main/scala/mesosphere/mesos/TaskBuilder.scala#L175).  There is also an [open issue](https://github.com/mesosphere/marathon/issues/375) for exposing arbitrary (user-defined) resources via the Marathon API.