Documentation added

@imlucas 

The file design.md includes the design of the implementation of random data generator (which can later be extended to dataset). 
I copied over the problem statement and implementation plan just as reference. No modification was made.
I was looking at the datasets hosted on Amazon AWS. Except those with ~TB size, there is a sample million song dataset that stores artists' information (I was so glad to find a portable one), but it turns out be in h5 format. I should probably not limit my scope to aws. There are plenty of portable ones elsewhere, such as citibikenyc.