Storing data sets used in the tutorials

[InessaPawson]

Document in README.md what options were considered for storing data sets used in the NumPy tutorials and why GitHub was selected as the optimal (interim) solution.

[rootsmusic]

@InessaPawson I think you're referring to the "Data sets" section in NEP 44.

[bsipocz]

@rootsmusic - we have datasets that don't fit those minimal datasets available through the methods listed in NEP44.