Clarified current Cordova version support
Clarified current Cordova version support

Added version support + Cordova 6 ticket reference. Made requirements section more prominent.
Laureckis/cordova-plugin-vuforia-background,thisisbd/cordova-plugin-vuforia,thisisbd/cordova-plugin-vuforia,happinov/cordova-plugin-vuforia,thisisbd/cordova-plugin-vuforia,mattrayner/cordova-plugin-vuforia,Laureckis/cordova-plugin-vuforia-background,happinov/cordova-plugin-vuforia,mattrayner/cordova-plugin-vuforia