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.)
Locale codes are a superset of language tags/codes. The BCP 47 spec is a language code, plus optional script, plus optional region, plus optional extensions and variants.

Our experience is with {lang}-{region}, as `en-US` is different than `en-GB` in ways that I'm sure frustrate the Brittish :)