Feature Request: Updated doc to use hieradata for sources

[kennyg]

This is a feature request to update the documentation to use hieradata for the r10k source configurations. Currently, we have the r10k class on our master and it drops a /etc/r10k.yaml with configuration data. I think it would be easier to maintain the r10k sources if it was externalized in hieradata common file, along with the other r10k config options that are being set.

[acidprime]

So I often only don't do this as hieradata is usually being sync'ed via r10k and so the first run is manual. I suppose I can just document that though

[acidprime]

I have updated the README with prefixing which should show how this would work
https://github.com/acidprime/r10k#prefixes

feel free to re-open if this is not sufficient.