Update README.md

Added note about silent-notifications.

Author: tingham

README.md

@@ -9,6 +9,9 @@ Official Push adapter for parse-server
 
 See [parse-server push configuration](https://github.com/ParsePlatform/parse-server/wiki/Push)
 
+## Silent Notifications
+
+If you have migrated from parse.com and you are seeing situations where silent (newsstand-like presentless) notifications are failing to deliver please ensure that your payload is setting the content-available attribute to Int(1) and not "1" This value will be explicitly checked.
 
 ### see more logs