Update README.md (#343)

Update instructions to cover iOS10 and below

Author: sattaman

README.md

@@ -293,6 +293,14 @@ Make `AppDelegate` conform to `RNAppAuthAuthorizationFlowManager` with the follo
 + @property(nonatomic, weak)id<RNAppAuthAuthorizationFlowManagerDelegate>authorizationFlowManagerDelegate;
 ```
 
+Add the following code to `AppDelegate.m` (to support iOS 10 and below)
+
+```diff
++ (BOOL)application:(UIApplication *)app openURL:(NSURL *)url options:(NSDictionary<NSString *, id> *) options {
++  return [self.authorizationFlowManagerDelegate resumeExternalUserAgentFlowWithURL:url];
++ }
+```
+
 #### Integration of the library with a Swift iOS project
 
 The approach mentioned above should also be possible to employ with Swift. In this case one should have to import `RNAppAuth`