add reference to react-testing-library

Because it will help engineers write better tests with react.

content/docs/addons-test-utils.md

@@ -20,6 +20,8 @@ var ReactTestUtils = require('react-dom/test-utils'); // ES5 with npm
 > 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 utility to use together with Jest, or any other test runner, it's worth checking out: [http://airbnb.io/enzyme/](http://airbnb.io/enzyme/)
+>
+> Alternatively, there is another testing utility called react-testing-library. This is a simple and small utility that helps your tests give you confidence your components are working by encouraging you to write tests that use your components as the end users use them. It also works with any test runner: [https://git.io/react-testing-library](https://git.io/react-testing-library)
 
  - [`Simulate`](#simulate)
  - [`renderIntoDocument()`](#renderintodocument)