Make sure we attribute and credit correctly

[JamesOwers]

Need to go through datasets we are using and packages and check the requirements on their use with respect to licences - we may need to attribute, and at minimum, I think we should add an attributions section to the main README

[apmcleod]

Piano-midi reqs are here: http://www.piano-midi.de/copy.htm. The standard is as a url footnote in the paper.

[apmcleod]

The paper cites pandas, pretty_midi, and mir_eval.

[apmcleod]

The paper includes a footnote link for PPDD, and cites LMD (where it draws data from).

[JamesOwers]

I'll make sure these are all added to the relevant README.md