update start guide (#800)

add didReceiveRemoteNotification

Co-authored-by: DanielEliraz <[email protected]>

Author: weihangChen

website/docs/docs/getting-started.md

@@ -73,6 +73,12 @@ And add the following methods to support registration to `AppDelegate.m`:
 }
 ```
 
+```objectivec
+- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
+  [RNNotifications didReceiveBackgroundNotification:userInfo withCompletionHandler:completionHandler];
+}
+```
+
 ### Android
 
 For Android installation, please refer to the [Android installation doc](installation-android.md) where you can find detailed step on how to start using Google's FCM service.