Update documentation

Rewrite the README. Expand it and attempt to more clearly outline how to use
katello-rapidnode.
This is unfortunate. I'm reserving some systems right now to test things out and double-check whether this is really necessary. I've submitted this PR anyway, though, because I think that this document represents a significant improvement even with this FIXME. And if I hold off until this is fixed, who knows how much longer this will be. I've already had this commit sitting in my repository for 15 days.