Add CONTRIBUTING.md

[jbmoelker]

The README's Contributing section is out of date (fastatic module is now a dep in package.json). I think this would be a good moment to add a CONTRIBUTING.md. Also to describe the aim of the project. As I see issues dripping in (be it in voorhoede/fastatic to add features to the app. And I think it would be good to have a motivation to consider those by.