Add link to data document

[andersonfrailey]

Add a link to the data description doc to TaxBrain. Full text is:

TaxBrain now offers users two data options: CPS and PUF. Both datasets contain tax unit level data, but they do not have all of the same variables. As a result, you will not be able to change some policy parameters, depending on which dataset you choose. A more detailed breakdown of the differences between the file can be found here.

cc @hdoupe @MattHJensen

[hdoupe]

LGTM. Here's what the top of the TaxBrain inputs page looks like now: