[ScrollView] Document onScrollBeginDrag and onScrollEndDrag (#99)

Author: ExplodingCabbage

docs/scrollview.md

@@ -31,6 +31,8 @@ This is where `FlatList` comes into play. `FlatList` renders items lazily, just
 - [`onMomentumScrollBegin`](scrollview.md#onmomentumscrollbegin)
 - [`onMomentumScrollEnd`](scrollview.md#onmomentumscrollend)
 - [`onScroll`](scrollview.md#onscroll)
+- [`onScrollBeginDrag`](scrollview.md#onscrollbegindrag)
+- [`onScrollEndDrag`](scrollview.md#onscrollenddrag)
 - [`pagingEnabled`](scrollview.md#pagingenabled)
 - [`refreshControl`](scrollview.md#refreshcontrol)
 - [`removeClippedSubviews`](scrollview.md#removeclippedsubviews)
@@ -190,6 +192,26 @@ Fires at most once per frame during scrolling. The frequency of the events can b
 
 ---
 
+### `onScrollBeginDrag`
+
+Called when the user begins to drag the scroll view.
+
+| Type     | Required |
+| -------- | -------- |
+| function | No       |
+
+---
+
+### `onScrollEndDrag`
+
+Called when the user stops dragging the scroll view and it either stops or begins to glide.
+
+| Type     | Required |
+| -------- | -------- |
+| function | No       |
+
+---
+
 ### `pagingEnabled`
 
 When true, the scroll view stops on multiples of the scroll view's size when scrolling. This can be used for horizontal pagination. The default value is false.