Update README.md

1. Copy www/PushNotification.js into the project's www directory

is not needed, PushNotification.js is located at www/plugins/com.urbanairship.phonegap.PushNotification/www/PushNotification.js and this file is called by cordova_plugin.js

1. Require the PushNotification module `var PushNotification = require('<Path to PushNotification.js>')`

returns an error of 'require is not defined', but works fine with cordova.require.  Not quite sure if this was changed in a updated version of Cordova

1. Require the PushNotification module `var PushNotification = cordova.require('<Path to PushNotification.js>')`