added doc for image resizeMode

Summary:
based on 62e8ddc205/ReactAndroid/src/main/java/com/facebook/react/views/image/ImageResizeMode.java  and http://developer.android.com/reference/android/widget/ImageView.ScaleType.html
Closes https://github.com/facebook/react-native/pull/4799

Reviewed By: svcscm

Differential Revision: D2762795

Pulled By: androidtrunkagent

fb-gh-sync-id: 2906c9f334f0afd4d58d4864a2b8472a5a871fc0

Libraries/Image/Image.ios.js

@@ -102,6 +102,17 @@ var Image = React.createClass({
     /**
      * Determines how to resize the image when the frame doesn't match the raw
      * image dimensions.
+     *
+     * 'cover': Scale the image uniformly (maintain the image's aspect ratio)
+     * so that both dimensions (width and height) of the image will be equal
+     * to or larger than the corresponding dimension of the view (minus padding).
+     *
+     * 'contain': Scale the image uniformly (maintain the image's aspect ratio)
+     * so that both dimensions (width and height) of the image will be equal to
+     * or less than the corresponding dimension of the view (minus padding).
+     *
+     * 'stretch': Scale width and height independently, This may change the
+     * aspect ratio of the src.
      */
     resizeMode: PropTypes.oneOf(['cover', 'contain', 'stretch']),
     /**