document that CameraRoll is not implemented on Android

Summary: The CameraRoll API shows up "unsuffixed" in the API list in the docs, so it is confusing to find that it is unimplemented on Android.
Closes https://github.com/facebook/react-native/pull/3901

Reviewed By: svcscm

Differential Revision: D2625837

Pulled By: mkonicek

fb-gh-sync-id: 4e5c00d260a30840a12fd9be0409c354d2052642

Libraries/CameraRoll/CameraRoll.js

@@ -119,6 +119,8 @@ class CameraRoll {
   /**
    * Saves the image to the camera roll / gallery.
    *
+   * The CameraRoll API is not yet implemented for Android.
+   * 
    * @param {string} tag On Android, this is a local URI, such
    * as `"file:///sdcard/img.png"`.
    *