Note that MapView is not yet available on Android

Summary:
The MapView component is not labelled as iOS-only, but it is. It took me a bit of digging to figure out that the Android source code hasn't been released. Found it in this [SO answer](http://stackoverflow.com/questions/32625259/no-view-manager-defined-for-class-rctmap) in the end, and it is documented [here](http://facebook.github.io/react-native/docs/known-issues.html#views), but it'd be nice if it said that directly in the MapView docs. (The mention of the Android-specific prop `active` put me off the trail for a bit.)
Closes https://github.com/facebook/react-native/pull/7361

Differential Revision: D3426119

fbshipit-source-id: 7bd210373a223a5dd756d3f095c6d1a3d8788b03

Libraries/Components/MapView/MapView.js

@@ -35,6 +35,15 @@ export type AnnotationDragState = $Enum<{
   ending: string;
 }>;
 
+/**
+ * A component for displaying embeddable maps and annotations using the native
+ * iOS MKMapView class. The Android version is not currently available in the
+ * open source React Native project, but you can use Leland Richardson's
+ * cross-platform and more feature-complete
+ * [react-native-maps](https://github.com/lelandrichardson/react-native-maps)
+ * instead.
+ */
+
 const MapView = React.createClass({
 
   mixins: [NativeMethodsMixin],