Added documentation for border[Top|Right|Bottom|Left]Width style attributes.

docs/docs/styles.md

@@ -158,6 +158,10 @@ overflow: 'hidden' | 'visible';
 **Borders**
 ```javascript
 borderWidth: number;
+borderTopWidth: number;
+borderRightWidth: number;
+borderBottomWidth: number;
+borderLeftWidth: number;
 borderColor: color;
 borderStyle: 'solid' | 'dotted' | 'dashed' | 'none';
 borderRadius: number;  // Sets all four border radius attributes; value is animatable