Update README to include onChangeTab

Author: brentvatne

README.md

@@ -147,6 +147,7 @@ module.exports = CustomTabBar;
   the tab bar. The component has `goToPage`, `tabs`, `activeTab` and
   `ref` added to the props, and should implement `setAnimationValue` to
   be able to animate itself along with the tab content.
+- **`onChangeTab`** _(Function)_ - function to call when tab changes, should accept 1 argument which is an Object containing two keys: `i`: the index of the tab that is selected, `ref`: the ref of the tab that is selected
 - **`edgeHitWidth`** _(Integer)_ - region (in pixels) from the left & right edges of the screen that can trigger swipe. Default is 30, which is the same as the swipe-back gesture on iOS.
 - **`springTension`** _(Integer)_ - a number between `1` and `100`, controls the amount of tension on the spring that guides the scroll animation - see [example](http://facebook.github.io/rebound-js/examples/#graph-canvas).
 - **`springFriction`** _(Integer)_ - a number between `1` and `30`, controls the amount of friction on the spring that guides the scroll animation - see [example](http://facebook.github.io/rebound-js/examples/#graph-canvas).