Missed showLabel props description (react-navigation#169)

Author: edzh1

docs/bottom-tab-navigator.md

@@ -27,6 +27,7 @@ The route configs object is a mapping from route name to a route config.
   * `inactiveTintColor` - Label and icon color of the inactive tab.
   * `inactiveBackgroundColor` - Background color of the inactive tab.
   * `showLabel` - Whether to show label for tab, default is true.
+  * `showIcon` - Whether to show icon for tab, default is true.
   * `style` - Style object for the tab bar.
   * `labelStyle` - Style object for the tab label.
   * `tabStyle` - Style object for the tab.
@@ -72,4 +73,4 @@ Callback to handle tap events; the argument is an object containing:
 * the `scene: { route, index }` that was tapped
 * the `jumpToIndex` method that can perform the navigation for you
 
-Useful for adding a custom logic before the transition to the next scene (the tapped one) starts.
+Useful for adding a custom logic before the transition to the next scene (the tapped one) starts.