When I first configured react-native-fcm I missed the compile project(':react-native-fcm') line because it's not stated as change in the diff viewer. corrected this

Update README to remove the line that says to uncomment the "use_frameworks!" line.

Can be misleading.

Will save users compilation error

* fixed documentation

* added .unsubscribe() method

* fix

* fix

* remove instead of unsubscribe

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

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