I was having build problems and was not able to use the underlying `FIRMessaging` object until I made this change.

This is just to provide a brief usage example and note about sending
upstream messages with `react-native-fcm`

Android notification + data push notifications when the app is in the foreground only passes the data packet through, this changes it to pass the notification data too

I was having build problems and was not able to use the underlying `FIRMessaging` object until I made this change.

In conforming with the method signatures as is the case with the
android version, I limited the arguments for `send()` to just
`senderId` and `payload`. This can still be improved upon by adding
`sendDataMessageSuccess` and `sendDataMessageFailure` to events being
fired across the bridge to Js.

* Fix iOS 10 specific documentation
* Fix typos/spelling-errors