Document supported versions of selenium

[damiencourousse]

hello,

Please could you document the versions of selenium that hs-webdriver supports?
i.e., I could run the beginner example, but only with chrome / chromedriver, and with version 2 of selenium (selenium-server-standalone-2.53.1.jar).

As reported in other issues (e.g. #126) it looks like selenium-version 3 is not yet supported.

thank you!