add note for mixins on es6

Author: sunnylqm

docs/Timers.md

@@ -74,4 +74,6 @@ var Component = React.createClass({
 });
 ```
 
-We strongly discourage using the global `setTimeout(...)` and recommend instead that you use `this.setTimeout(...)` provided by react-timer-mixin. This will eliminate a lot of hard work tracking down bugs, such as crashes caused by timeouts firing after a component has been unmounted.
+This will eliminate a lot of hard work tracking down bugs, such as crashes caused by timeouts firing after a component has been unmounted.
+
+However, `TimerMixin` and any other mixins *can not be used on ES6*. You need to use some helper like [react-mixin](https://github.com/brigand/react-mixin) to migarate existing mixins to ES6 classes.