Add initial resources.md for discussion

The first draft of a Kubernetes resource model.
Is there a facility for encoding other metadata about a particular resource type?  I'm particularly interested in the base units.

For example, how do I know whether `resources.kubernetes.io/cpu` is measured in `cpu-seconds/second`, `cpu-shares` or some other thing?

This can happen outside of this protocol ([as in Mesos](https://github.com/apache/mesos/blob/master/include/mesos/mesos.proto#L370)), which might not be a problem in practice.