- added available SceneConfigs to Navigator docs

Summary:At the beginning of the navigator docs it says " See Navigator.SceneConfigs for default animations and more info on scene config options."

but then this is the only information available:

<img width="658" alt="screen shot 2016-03-23 at 2 40 49 am" src="https://cloud.githubusercontent.com/assets/1247834/13977670/733cdaa6-f0a1-11e5-92e6-fc98725f65e0.png">

And the only way of knowing about the available options is to look at the source code. I think a lot of people will appreciate this...

If you think this is too much, maybe adding a link to the source code would help.
Closes https://github.com/facebook/react-native/pull/6599

Differential Revision: D3088592

fb-gh-sync-id: f11025b76441e9394ce42ed6c796fa1ace3e8c27
fbshipit-source-id: f11025b76441e9394ce42ed6c796fa1ace3e8c27

Libraries/CustomComponents/Navigator/Navigator.js

@@ -195,6 +195,20 @@ var Navigator = React.createClass({
      * ```
      * (route, routeStack) => Navigator.SceneConfigs.FloatFromRight
      * ```
+     *
+     * Available options are:
+     *
+     *  - Navigator.SceneConfigs.PushFromRight (default)
+     *  - Navigator.SceneConfigs.FloatFromRight
+     *  - Navigator.SceneConfigs.FloatFromLeft
+     *  - Navigator.SceneConfigs.FloatFromBottom
+     *  - Navigator.SceneConfigs.FloatFromBottomAndroid
+     *  - Navigator.SceneConfigs.FadeAndroid
+     *  - Navigator.SceneConfigs.HorizontalSwipeJump
+     *  - Navigator.SceneConfigs.HorizontalSwipeJumpFromRight
+     *  - Navigator.SceneConfigs.VerticalUpSwipeJump
+     *  - Navigator.SceneConfigs.VerticalDownSwipeJump
+     *
      */
     configureScene: PropTypes.func,