Update README.md

Author: jmadler

README.md

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-[OneSignal](https://onesignal.com/) is a free push notification service for mobile apps. This SDK makes it easy to integrate your Flutter iOS and/or Android apps with OneSignal.
+[OneSignal](https://onesignal.com/) is a free email, sms, push notification, and in-app message service for mobile apps. This SDK makes it easy to integrate your Flutter iOS and/or Android apps with OneSignal.
 
 <p align="center"><img src="https://app.onesignal.com/images/android_and_ios_notification_image.gif" width="500" alt="Flutter Notification"></p>