MCO repo doc for "OCP layering Phase 0" with brief examples/tradeoffs/warnings

We need something in our repo /docs that we can point people to that briefly explains how to use "layering features" via the MCO in OCP ( well, and with the understanding that OKD also uses the MCO ). 

Maybe this ends up in its own repo like https://github.com/coreos/coreos-layering-examples eventually, maybe it doesn't.

I'm thinking something like https://github.com/openshift/machine-config-operator/blob/layering/docs/DemoLayering.md back from when we did the layering branch, but actually matching what we have in our main branch

This is separate but probably related to what Colin started in the Docs Tracker.