Add docs for onMomentumScrollBegin

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Closes https://github.com/facebook/react-native/pull/15158

Differential Revision: D5479401

Pulled By: shergin

fbshipit-source-id: d4864e1630a36deb1a227c1b6242255ac1f788e6

Libraries/Components/ScrollView/ScrollView.js

@@ -235,6 +235,10 @@ const ScrollView = createReactClass({
      * @platform ios
      */
     minimumZoomScale: PropTypes.number,
+    /**
+     * Called when the momentum scroll starts (scroll which occurs as the ScrollView glides to a stop).
+     */
+    onMomentumScrollBegin: PropTypes.func,
     /**
      * Called when the momentum scroll ends (scroll which occurs as the ScrollView glides to a stop).
      */