provide link to instructions for building for iOS

Fixes #https://github.com/flutter/flutter/issues/3689

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f797cc6..1182703 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -92,6 +92,10 @@
 `android/AndroidManifest.xml` file for the Flutter app you are using
 to test the engine.
 
+### iOS
+
+See [this wiki page](https://github.com/flutter/engine/wiki/Flutter-Apps-on-iOS).
+
 ### Desktop (Mac and Linux), for tests
 
  * `gclient sync` to update your dependencies.