Outline of a paper on Clawpack 5.x

It would be good to publish a brief paper outlining changes to Clawpack from 4.x to 5.x, as a possible suggested citation for users of Clawpack.

Motivated by discussions on how to cite software at PDESoft 2014, see also 
  https://github.com/uwescience/citing_software

We can at least post this paper on arXiv, possibly submit to ??.