Add a README.md for datasets?

[loganjohnson0]

I was thinking more about some of the datasets in the data/ folder. One thing I noticed is that the datasets are not very well described or named. Coming from the outside of the group, I don't quite understand these datasets or what the values of each of these datasets really represent.

I see a prior issue related to #127 documenting datasets. I couldn't find any related changes in the repository to documentation.

Any thoughts on that? I think this documentation would be helpful with #329.

[DrLynTaylor]

Totally Agree :) If you have any ideas of how to make this better, feel free to suggestion something. Thanks.