Create papers.md

[maximilianh]

starting a list of just papers, so someone could collect the data one day

Thank you for contributing to easy-data! Please make sure your markdown file explaining the dataset has the following:

• a description of the dataset including a link to the appropriate publication or
  reference.
• direct links to download the count matrix (in the form of an easy-to-load file, like a rds file containing an sparse matrix for R and an AnnData hdf5 file or
  a mtx file for python).
• direct links to download the metadata (in a csv with rows indexed by cell names).
• sample loading code for R and python.

[jacksonloper]

Interesting idea! There are so many places we might be able to get the data from someday though... not sure if this really fits the easy-data mission :). But I'm curious what other folks think.