added directive watcher note

Author: johnpapa

README.md

@@ -569,7 +569,7 @@ Many of my styles have been from the many pair programming sessions [Ward Bell](
 
     -Note: There are many naming options for directives, especially since they can be used in narrow or wide scopes. Choose one the makes the directive and it's file name distinct and clear. Some examples are below, but see the naming section for more recommendations.
 
-- **Limit DOM Manipulation**: When manipulating the DOM directly, use a directive. If alternative ways can be used such as using CSS to set styles or the [animation services](https://docs.angularjs.org/api/ngAnimate), Angular templating, [`ngShow`](https://docs.angularjs.org/api/ng/directive/ngShow) or [`ngHide`](https://docs.angularjs.org/api/ng/directive/ngHide), then use those instead. 
+- **Limit DOM Manipulation**: When manipulating the DOM directly, use a directive. If alternative ways can be used such as using CSS to set styles or the [animation services](https://docs.angularjs.org/api/ngAnimate), Angular templating, [`ngShow`](https://docs.angularjs.org/api/ng/directive/ngShow) or [`ngHide`](https://docs.angularjs.org/api/ng/directive/ngHide), then use those instead. For example, if the directive simply hide and shows, use ngHide/ngShow, but if the directive does more, combining hide and show inside a directive may improve performance as it reduces watchers. 
 
     *Why?*: DOM manipulation can be difficult to test, debug, and there are often better ways (e.g. CSS, animations, templating)