Add a contributing section

[stof]

This page (or pages) should explain how to report issues (which repo), and how to contribute to the repo, including new features impacting drivers (which require updating 2 testsuites in Mink to cover drivers)

[aik099]

I think it should also explain how to setup workspace to do this:

1. clone mink repo
2. clone each driver repo
3. install composer dependencies for all of them
4. symlink /vendor/behat/mink in drivers with cloned Mink repo so they will share test suite changes