Durable local storage proposal

This is a proposal for #598


If this is to be a general doc on volumes, I think you need ease in more gently.

A Volume is a directory, possibly with some data in it, which is exposed to a Container.  The file system that a container experiences is a combination of its Docker image plus any volumes mounted into the container.  This is similar in most regards to what Docker knows as Volumes, but is different in some of teh details of how Volumes are defined and managed.