Document an example for how the snapshot controller library will be used by volume plugins

[harsh-px]

As discussed in the snapshot working group meeting on May 30th, the snapshot controller will be provided as a library which other volume plugins can import and implement the interfaces. So I'm guessing this is going to be similar to the model followed by the external hostpath provisioner.

I'll be great to have a similar example for the snapshot controller so other volume plugin developers can get an idea on how to use this.