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.
People who want to develop using mono certainly don't need to be shown how to run `cd`. If anything, telling them they need xbuild is more useful, but I think "run make" is sufficient.