Tweaked the README to be a bit more friendly and detailed

I thought the README could use some love, so I added some details about licensing and also detailed out procedures for downloading and building the project.

In addition I added a bit to the "What do you get" paragraph and tried to tighten it all up some.
Terminal.app? I didn't know SignalR ran on a Mac - that's cool :).  I don't think we can assume anything along these lines - they might just download the ZIP file and go from there. In every project I've run that's MS-related I've had first-time Git users who don't care to clone anything or use any type of command line.