Add deprecation warning to MapView

Libraries/Components/MapView/MapView.js

@@ -86,6 +86,19 @@ export type AnnotationDragState = $Enum<{
 
 const MapView = React.createClass({
 
+  componentWillMount: function() {
+    console.warn(
+      'MapView is now deprecated and will be removed from React Native in version 0.42. ' +
+      'Please use the react-native-maps module which is more feature complete ' +
+      'and works on Android too: https://github.com/airbnb/react-native-maps\n' +
+      'It is actively maintained and widely used.\n\n' +
+      'Once MapView is removed from React Native in v0.42, we will release the ' +
+      'code as deprecated-react-native-ios-mapview. You will be able to ' +
+      'continue using that and migrate to react-native-maps your own pace later.\n\n' +
+      'For more info, check out https://github.com/facebook/react-native/pull/10500'
+    );
+  },
+
   mixins: [NativeMethodsMixin],
 
   propTypes: {