Adding MinkPhantomJSDriver to the documentation

[jcalderonzumba]

Adding MinkPhantomJSDriver to the documentation via new section.
Driver is already integrated with mink/driver-test-suite and has been change to address all the topics discussed with @stof.

[stof]

I don't think we should be maintaining the doc for third-party drivers ourselves. You are the one knowing the driver and so you should be maintaining its doc.
The list of third-party drivers should have external links to your doc instead.

[jcalderonzumba]

You have a good point here, i'll create a phantomjsdriver.readthedocs.org page with the Driver info and link it here then.
I already have a gastonjs.readthedocs.org documentation so it makes senses to have a similar doc site for phantomjsdriver.
I'll update the PR once the new doc page is up and running.

[stof]

if it is small, you could also use github itself to host the documentation (write it in markdown in this case though, as their rST renderer is not as good as their markdown one) (putting the doc in the /docs folder of the repo for instance)

[jcalderonzumba]

Yes, for the moment is small and mostly everything that i have written in this PR is contained in the README.md of the repo.

[stof]

IMO, the third-party drivers could be a section at the end of the drivers page instead of a separate page containing only the list, which has a single element for now.
This could be changed in the future if we have tens of third-party drivers, but I doubt it will be the case.