Rewrite of the main README.

Have done a rewrite. Removed what was essentially my original specification with more of a functional/conceptual description. Am also working on a simple worked example that shows off the CLI (stay tuned for another PR).
It is, and should go in a more comprehensive developer/user guide ... when we have one. The only reason I've added it here is that without this bit of information, you can't get anything to work in local mode on your laptop if you're running OS X.