Update the readme.
Update the readme.

Importing `RCTOneSignal.h` in `AppDelegate.m` is unnecessary
duplication since `AppDelegate.h` already includes it. Also, it would
be reasonable to write the header updates first, that way newcomers
won't get disturbed by XCode's errors notification (which you will get
if you edit `AppDelegate.m` first).

geektimecoil/react-native-onesignal,geektimecoil/react-native-onesignal,maxcodes/react-native-onesignal,eriksape/react-native-onesignal,maxcodes/react-native-onesignal,maxcodes/react-native-onesignal,geektimecoil/react-native-onesignal,eriksape/react-native-onesignal,eriksape/react-native-onesignal