Updated README.md.
Updated README.md.

- Added a note to indicate that the GameKit.framework is required.
- Updated configuration instructions from referencing Cordova.plist to
  config.xml.

Wizcorp/cordova-plugin-gamecenter,Wizcorp/cordova-plugin-gamecenter,Wizcorp/cordova-plugin-gamecenter,Wizcorp/cordova-plugin-gamecenter