Add a Citation Recommendation to the Readthedocs documentation

[dougbrn]

We've received multiple questions from users asking how to cite/credit stistools in their work. At the time, we don't have a DOI, but we have been recommending people cite the readthedocs page. The documentation citation should look roughly like this: “Hack, W., et al. 2018, STIS Python User Tools (Baltimore, MD: STScI), https://stistools.readthedocs.io/en/latest/”. We should add this to the readthedocs documentation.

[ladsantos]

If we want a DOI, there's always the option of publishing it on Zenodo.