add stubby OS & browser compatibility table to docs

This is a stop-gap until #277 gets fixed properly by the Ratchet Team.
Per #662, iOS<7.1 isn't supported.
Other OSes and browsers it would be good to eventually make a support decision about: BlackBerry, UC Browser, Dolphin, webOS, bada.
Styling of the table is suboptimal and cribbed from Bootstrap because IANADesigner. Consider it an incentive to finish filling out the table.  :-P
FYI: @connors