Add links/notes about installing Ruby

I've written to a few people suggesting that someone should start a project called installingruby.org, and that all the railsbridges, the hartls, the bootcamps, the railsgirlses of the world can contribute to keeping that up to date instead of having all these rapidly out-of-date resources.

Hartl says it best, quoting Paul Graham:

> Practically all the software in the world is either broken or very difficult to use. So users dread software. They’ve been trained that whenever they try to install something, or even fill out a form online, it’s not going to work. I dread installing stuff, and I have a Ph.D. in computer science.
> —Paul Graham, in Founders at Work by Jessica Livingston