Units user story

[lilyminium]

User story

As an infrequent user of MDAnalysis,
I want to easily know what the units of MDAnalysis are,
so that I don't accidentally mix up units in my analyses.

Additional context

There are units somewhere in the docs, but they should be
up front and center as one of the first things to know about MDAnalysis!

[lilyminium]

This should probably go in https://github.com/MDAnalysis/UserGuide/blob/develop/doc/source/examples/quickstart.ipynb (the quick start guide) or the home page (https://github.com/MDAnalysis/UserGuide/blob/develop/doc/source/index.rst)