Note on 'ruby-install'

Add section about ruby-install.

Although subjective, I feel it merits inclusion. It's well-written, simple, works on a multitude of OSes and is liked by active members in the ruby community.

What do you think, is this something that could be added to the site?

> Written by Postmodern, it's basically the simplest possible thing that can work.
> -- Steve Klabnik

> I like the approach you're taking. Curious to see how it plays out.
> -- Sam Stephenson of rbenv

*Note: I'm not affiliated with ruby-install, I just like it.*