Update NavigatorComparison.md

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"nobody who uses this has stepped up to take ownership for it yet" - react-native-navigation is a rewrite which is a fully featured
Closes https://github.com/facebook/react-native/pull/6888

Differential Revision: D3161511

fb-gh-sync-id: b3e0c1f6138fb40561cb542d8905bcc478f781cb
fbshipit-source-id: b3e0c1f6138fb40561cb542d8905bcc478f781cb

docs/NavigatorComparison.md

@@ -40,6 +40,7 @@ between the two.
 - Small, limited API makes it much less customizable than `Navigator` in its current form.
 - Development belongs to open-source community - not used by the React Native team on their apps.
   - A result of this is that there is currently a backlog of unresolved bugs, nobody who uses this has stepped up to take ownership for it yet.
+  - You may find an alternative in the community project [react-native-navigation](https://github.com/wix/react-native-navigation) which replaces `NavigatorIOS`.
 - Wraps UIKit, so it works exactly the same as it would on another native app. Lives in Objective-C and JavaScript.
   - Consequently, you get the animations and behavior that Apple has developed.
 - iOS only.