Merge pull request #442 from davidnguyen179/reference-test-renderer/note__react-test-render_react_react-dom

react-test-render/Cannot read property 'ReactCurrentOwner' of undefined

content/docs/add-react-to-an-existing-app.md

@@ -40,6 +40,10 @@ npm install --save react react-dom
 
 Both Yarn and npm download packages from the [npm registry](http://npmjs.com/).
 
+>  Note:
+>
+>  To prevent potential incompatibilities, all react packages should use the same version. (This includes `react`, `react-dom`, `react-test-renderer`, etc.)
+
 ### Enabling ES6 and JSX
 
 We recommend using React with [Babel](http://babeljs.io/) to let you use ES6 and JSX in your JavaScript code. ES6 is a set of modern JavaScript features that make development easier, and JSX is an extension to the JavaScript language that works nicely with React.