react-navigation#186 add a section to explain the use case

Author: Albert-Gao

docs/bottom-tab-navigator.md

@@ -20,7 +20,7 @@ The route configs object is a mapping from route name to a route config.
 * `order` - Array of routeNames which defines the order of the tabs.
 * `paths` - Provide a mapping of routeName to path config, which overrides the paths set in the routeConfigs.
 * `backBehavior` - Should the back button cause a tab switch to the initial tab? If yes, set to `initialRoute`, otherwise `none`. Defaults to `initialRoute` behavior.
-* `tabBarComponent` - Optional, override component to use as the tab bar. You need to import `BottomTabBar` from `react-navigation-tabs` in order to receive the passing `props`.
+* `tabBarComponent` - Optional, override component to use as the tab bar.
 * `tabBarOptions` - An object with the following properties:
   * `activeTintColor` - Label and icon color of the active tab.
   * `activeBackgroundColor` - Background color of the active tab.
@@ -47,6 +47,25 @@ tabBarOptions: {
 }
 ```
 
+If you want to customize the `tabBarComponent`:
+
+```js
+import { createBottomTabNavigator, BottomTabBar } from 'react-navigation-tabs';
+
+const TabBarComponent = (props) => (<BottomTabBar {...props} />);
+
+const TabScreens = createBottomTabNavigator(
+  {
+    tabBarComponent: props =>
+      <TabBarComponent
+        {...props}
+        style={{ borderTopColor: '#605F60' }}
+      />,
+  },
+);
+```
+
+
 ## `navigationOptions` for screens inside of the navigator
 
 #### `title`