Add initial resources.md for discussion

The first draft of a Kubernetes resource model.
@ConnorDoyle It matters during updates, yes.  It also matters for more "advanced" scenarios (which we don't need to support yet) where a user intentionally overcommits inside a pod (e.g. an opportunistic cache) or where we start adding shared resources that are are accounted to the pod, but not any one container (this is the part that we do not yet know how we want to model with Docker).