Update addons-test-utils.md (reactjs#3660)

Co-authored-by: Sebastian Silbermann <[email protected]>

content/docs/addons-test-utils.md

@@ -20,8 +20,10 @@ var ReactTestUtils = require('react-dom/test-utils'); // ES5 with npm
 > Note:
 >
 > We recommend using [React Testing Library](https://testing-library.com/react) which is designed to enable and encourage writing tests that use your components as the end users do.
->
-> Alternatively, Airbnb has released a testing utility called [Enzyme](https://airbnb.io/enzyme/), which makes it easy to assert, manipulate, and traverse your React Components' output.
+> 
+> For React versions <= 16, the [Enzyme](https://airbnb.io/enzyme/) library makes it easy to assert, manipulate, and traverse your React Components' output.
+
+
 
  - [`act()`](#act)
  - [`mockComponent()`](#mockcomponent)