Image verification docs

[pop]

Related to customers running into problems caused by faulty image downloads.

Refs #760, #583, #736, and https://coreos.com/os/docs/latest/verify-images.html

I'm not 100% sure I found all of the references / places to include pointers to the verify-image doc. It's a pretty manual process.

[euank]

needs rebase

[zbwright]

@joshix what should we do about Elijah's open PRs?