Add a data dictionary/metadata file for the CSV headers

[tacman]

Please considering adding a metadata file that describes the CSV headers. #33 mentions that adding metadata is something you're working on, what's the status of that?

A file that describes the fields would make it easier to incorporate. For example. booleans in this file are represented by True and False, Dimensions probably follow a specific format that includes metric in parenthesis, Geography Type is likely a specific list, that kind of stuff. Otherwise, each developer has to manually inspect the data.

Thanks for your consideration.