Add documentation on using Emoji Shortcodes in Markdown text fields.
Review Request #9309 — Created Oct. 23, 2017 and submitted
This updates the Markdown documentation on what Emoji Shortcodes are and
how to use them. It lists a few examples of common ones that people
might want to use and then points to a well-maintained cheat sheet of
GitHub Emoji shortcodes (GitHub, sadly, does not provide their own).
Built the docs and looked through them. No build errors, and links worked.