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.)
Capitalize "message", use s/format/syntax/? -> "ICU Message syntax".