While writing the documention, I found that "GPG user" was less confusing
terminology than "GPG key," since you aren't really adding a "key"
to git-crypt, and git-crypt already uses "key" to refer to other concepts
(cf. the -k/--key-name options).
* format section headings, links and code snippets
* add .md file extension to trigger pretty rendering on Github
* standardize on lowercase typesetting for git-crypt
