first pass at fleshing out the guide

Given the nature of this PR, it might be easier to review/comment on a rendered version of the content. I've created a google doc out of this page:
https://docs.google.com/document/d/1vR1acWGkUWPw5L5mvAh8tRg0UY_bB8H-Ajfv7pd8fSg/edit#
(It's not a perfect copy.)
Also when writing these message, and assuming they are `.txt` files or something in their code blocks, I like this indentation style: https://github.com/yahoo/formatjs-site/blob/master/i18n/en-US.yaml