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.
This makes it seem like the Apache licence requires that you document any modifications you make to SignalR. I don't see the benefit in trying to summarize a fairly well known license in our README. I doubt our lawyers would like it either.