Updated README.md to include warning about API <= 22
Updated README.md to include warning about API <= 22

mesmotronic/cordova-plugin-fullscreen,mesmotronic/cordova-fullscreen-plugin,mesmotronic/cordova-fullscreen-plugin,mesmotronic/cordova-fullscreen-plugin,mesmotronic/cordova-plugin-fullscreen,mesmotronic/cordova-plugin-fullscreen