spec: state diagram needed for context w/ respect to snapshots

[jdef]

Link: https://github.com/container-storage-interface/spec/blob/master/spec.md#the-ready_to_use-parameter

There's a paragraph in the above section that talks about "freeze" and "thaw" actions w/ respect to a container life cycle and when snapshot operations are applied. It would be useful to:

1. Use more appropriate quoting for the terms "freeze" and "thaw" (vs. freeze, thaw since they're not API related)
2. Provide a state diagram to illustrate container life cycle (as managed by the CO) and when snapshot operations should be applied.

[xing-yang]

I'll take a look of this.