README for newbies

Hello!  I learned about this project through a [reddit post](http://www.reddit.com/r/rust/comments/2dtgad/rust_gamedev_news/) and couldn't help but try it out.  I know it's like super alpha right now but I wanted to contribute a better readme to help others interested in building/running the project.  I only have the prerequisites done and still have to work on the build section.  I didn't get to fix the build section just yet because I'm still hacking away at getting it to run via `cargo run`.

Thanks again for your awesome contribution!