[RFC] Update tutorial.md into GUIDE.md and add sections

The goals of this new resource is to succeed the Tutorial on the piston.rs site, guiding new users to
1. Install Rust and Cargo
2. Create a hello world project with Piston
3. Show them where to go afterwards

This serves as our hook for people interested in Piston. They can start making things immediately, and then use this as a foundation for growing their understanding of the diverse projects Piston encapsulates or relies on.

There's overlap with the information provided in the README, and for simplicity's sake, the info expressed here may fit better in the readme. 

!!! The installation instructions are not yet completed. 

For the Hello World section, I linked to the awesome Getting Started tutorial.

If someone provide installation instructions for Windows, including setup instructions for our Visual Studio stuff, I'm sure we'd see an influx of windows devs.

I'm hoping to be able to polish it up the over the weekend. Feedback and additions are very much appreciated!
I thought Cargo was installed with Rust?

Just pointing users to http://www.rust-lang.org will suffice.