doc: add need for USE_FULL_SCREEN_INTENT

Author: fabriziomoscon

README.md

@@ -20,7 +20,8 @@ To allow the library to show heads up notifications you must add the following l
 
 ```xml
     <!-- receive calls when the app is in the background-->
-    <uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>
+    <uses-permission android:name="android.permission.USE_FULL_SCREEN_INTENT" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
 
     <application
         ...