Add a CONTRIBUTING.md for people creating pull requests.

[martinpelikan]

Could I suggest adding a CONTRIBUTING.md file to the repository, which would cover general test setup and structure?

I was able to figure out how to run the tests by reading the TravisCI logs (installing Firefox-ESR, nose, etc.), but without your pointers I would have had some struggle figuring out where best to write the tests (or even that it was necessary for the PR to get merged).