turn tutorial.md into GUIDE.md and add content
add link to gfx-rs
add platform-agnostic installation links; clean up Hello World and Next Steps
remove RFC flag over Why Piston
fix indentation on nightlies links
Merge pull request #655 from bvssvni/master

Fix bug
Merge pull request #629 from mkcp/master

[RFC] Update tutorial.md into GUIDE.md and add sections
Adding self to the contributors table
Moved fps counter and drag controlled to piston-event

Closes https://github.com/PistonDevelopers/piston/issues/658