Add more acronym guidance based on writing guide

* Based on
http://alphagov.github.io/inside-government-admin-guide/creating-documen
ts/markdown.html#acronyms
* Include specifics about what should be listed (plurals, not
possessives) and caveats to avoid (spaces in markdown)
* Add a second acronym to example so that markdown looks like a list
(the asterisk begins to make more sense in this context)