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.
Hi @robconery , we should keep the license information as terse as possible so as to not create any confusion around what the license does or doesn't do, and how to reason about it. I think keeping the text at just this:

    SignalR is licensed under the [Apache 2.0 License](https://github.com/SignalR/SignalR/blob/master/LICENSE.md). 

will do the best service to the reader.