Write a Getting Started guide

[Cantido]

I like what I've done with the README and module docs so far, but I want a little more room to breathe. So I'll write a proper Getting Started guide, being inspired by the original Liberator's docs.

Other folks can be a huge help to me by going through the Getting Started guide I'm writing, and trying out the steps for themselves. Let me know if you run into an issue or if anything isn't clear. And make a pull request if you've got some docs you want to change.

My WIP lives at https://github.com/Cantido/liberator/blob/master/getting_started.md right now. Tell me what you think.