Add php-pushwoosh to the Notifications section

This add php-pushwoosh to the Notifications section, php-pushwoosh is a
well tested library with more than 1000 downloads / month.

This library is very stable and is widely used, so it can safely be
added to awesome-php.

README.md

 * [Notification Pusher](https://github.com/Ph3nol/NotificationPusher) - A standalone library for device push notifications.
 * [Notificato](https://github.com/mac-cain13/notificato) - A library for handling push notifications.
 * [Notificator](https://github.com/namshi/notificator) - A lightweight notification library.
+* [Php-pushwoosh](https://github.com/gomoob/php-pushwoosh) - A PHP Library to easily send push notifications with the Pushwoosh REST Web Services.
 
 ## Deployment
 *Libraries for project deployment.*