Update README.md (#225)

The `sendTokenToIntercom` method sends the Firebase Cloud Messaging (FCM) token to intercom for push notifications. However despite FCM can be used for both Android and iOS, Intercom only supports FCM tokens for Android and not for iOS, and that's why `sendTokenToIntercom` is [stubbed for iOS on this library](https://github.com/tinycreative/react-native-intercom/blob/374faecd21ffc46d2582ea4d6e97b2ae3f8f0211/iOS/IntercomWrapper.m#L44).

This generated a lot of confusion for us since we were expecting FCM to work cross-platform.

Also maybe `sendTokenToIntercom` should be renamed to `sendFCMTokenToIntercom`.

Author: Javier Cuevas

README.md

@@ -278,7 +278,7 @@ _onUnreadChange = ({ count }) => {
     Intercom.Notifications.WINDOW_DID_HIDE
 ```
 
-### Send FCM token directly to Intercom
+### Send FCM token directly to Intercom for push notifications (Android only)
 ```
 Firebase.messaging().getToken()
   .then((token) => {