Add skopeo information to the image promotion guide

[srang]

Skopeo is a good tool for managing images between repos, it would be good to cover its uses in the image promotion guide http://v1.uncontained.io/playbooks/continuous_delivery/image_promotion.html

[etsauer]

@srang good idea. @redhat-cop/cant-contain-this any takers?

[srang]

So I've got to write something up anyways, so if no one else takes it before then (shooting to have it done this week) I'll contribute that back here as well