Add a paragraph about third-party driver implementations

[stof]

Thanks to http://packanalyst.com/class?q=Behat\Mink\Driver\DriverInterface I found the implemetations of our DriverInterface in Packagist packages. Apart from several child classes of our drivers for specific testsuite needs (not always necessary for Mink 1.6 btw), this allowed me to detect 2 different implementations of a capybara-webkit driver, which was initially proposed in minkphp/Mink#221 (but never completed). I requested the status of these drivers, as they are both forcing to use Mink 1.5 currently.
I suggest adding section about third-party drivers in http://mink.behat.org/en/latest/guides/drivers.html once we get answers from the respective maintainers to know whether they want them to be listed (a prerequisite would be to make them compatible with Mink 1.6 and a nice-to-have would be to make them run the Mink 1.6+ driver testsuite):

• Status of the driver bertrandmoulard/pigeon-webkit#2
• Status of the driver nstojiljkovic/mink-capybara-webkit-driver#1

[aik099]

Maybe we can even move them to minkphp organization once they are adapted.

[stof]

Well, they can be moved to the minkphp organization only if their current maintainers continue to maintain them actively. I don't want to have yet another driver which will fall into our own workload because other maintainers stop working on them.

[aik099]

OK.

[stof]

another one: https://github.com/jcalderonzumba/MinkPhantomJSDriver (see minkphp/Mink#654)