Add Compactness definition

[thejessleigh]

The website should include a tooltip or section that defines terms like compactness. Also, having a quick definition in the README and a description of how and where that calculation happens would be very helpful. I think that could go in either the readme or in public/districts/pa/README.

Lastly, the README in public/districts/pa has some very useful information, but it's kind of buried in the file structure. I think this could either be moved to public/districts or it could be pulled out entirely into the top level README for easy access. :)