Update documentation

Rewrite the README. Expand it and attempt to more clearly outline how to use
katello-rapidnode.
The activation key doesn't provide the content view. Rather, it __can__ include products published in a content view. An activation key can be as minimal as a single subscription, without enabled products, or it can include numerous subscriptions and enabled products. In regards to whether it is required for a capsule or not, I am not experienced with capsules enough to say either way.