Mention Enzyme on the test utils page. (#6523)

docs/10.4-test-utils.md

@@ -12,6 +12,10 @@ next: clone-with-props.html
 var ReactTestUtils = require('react-addons-test-utils');
 ```
 
+> Note:
+> 
+> Airbnb has released a testing utility called Enzyme, which makes it easy to assert, manipulate, and traverse your React Components' output. If you're deciding on a unit testing library, it's worth checking out: [http://airbnb.io/enzyme/](http://airbnb.io/enzyme/)
+
 ### Simulate
 
 ```javascript