Improve citation guidelines

[mattwthompson]

We recently came across a paper that cited the toolkit as

AM1/BCC partial charges were assigned using the OpenFF toolkit with default
settings.

Unfortunately there are two (possibly three, soon) default settings. RDKit/AmberTools and OpenEye generate slightly different charges for many molecules (#447 and many other discussions) based on what is installed and licensed, and soon NAGL will introduce a third backend. Having code associated with paper(s) helps but doesn't necessarily solve this issue if it's not clear what packages are installed at runtime.

We can't enforce citation guidelines, but I suggest we should nudge people towards including critical information like the backend used for charge generation. Are there other details that are this important (read: text that is necessary to reproduce results, even when code is included)?