Add docs for View BorderWidth

Summary: Closes https://github.com/facebook/react-native/pull/3133

Reviewed By: @​svcscm

Differential Revision: D2502503

Pulled By: @mkonicek

Libraries/Components/View/ViewStylePropTypes.js

@@ -34,6 +34,11 @@ var ViewStylePropTypes = {
   borderBottomLeftRadius: ReactPropTypes.number,
   borderBottomRightRadius: ReactPropTypes.number,
   borderStyle: ReactPropTypes.oneOf(['solid', 'dotted', 'dashed']),
+  borderWidth: ReactPropTypes.number,
+  borderTopWidth: ReactPropTypes.number,
+  borderRightWidth: ReactPropTypes.number,
+  borderBottomWidth: ReactPropTypes.number,
+  borderLeftWidth: ReactPropTypes.number,
   opacity: ReactPropTypes.number,
   overflow: ReactPropTypes.oneOf(['visible', 'hidden']),
   shadowColor: ReactPropTypes.string,