Update parent/child communication tip

tips/14-communicate-between-components.md

@@ -38,3 +38,5 @@ React.renderComponent(
 ```
 
 Notice the use of `bind(this, arg1, arg2, ...)`: we're simply passing more arguments to `handleClick`. This is not a new React concept; it's just JavaScript.
+
+For communication between two components that don't have a parent-child relationship, you can set up your own global event system. Subscribe to events in `componentDidMount()`, unsubscribe in `componentWillUnmount()`, and when you receive an event call `setState()`.