add react-testing-library documentation/examples (#4679)

* add react-testing-library documentation/examples

* make react-testing-library a heading

* fix typo

packages/react-scripts/template/README.md

@@ -1428,6 +1428,48 @@ Import it in [`src/setupTests.js`](#initializing-test-environment) to make its m
 import 'jest-enzyme';
 ```
 
+#### Use `react-testing-library`
+
+As an alternative or companion to `enzyme`, you may consider using `react-testing-library`. [`react-testing-library`](https://github.com/kentcdodds/react-testing-library) is a library for testing React components in a way that resembles the way the components are used by end users. It is well suited for unit, integration, and end-to-end testing of React components and applications. It works more directly with DOM nodes, and therefore it's recommended to use with [`jest-dom`](https://github.com/gnapse/jest-dom) for improved assertions.
+
+To install `react-testing-library` and `jest-dom`, you can run:
+
+```sh
+npm install --save react-testing-library jest-dom
+```
+
+Alternatively you may use `yarn`:
+
+```sh
+yarn add react-testing-library jest-dom
+```
+
+Similar to `enzyme` you can create a `src/setupTests.js` file to avoid boilerplate in your test files:
+
+```js
+// react-testing-library renders your components to document.body,
+// this will ensure they're removed after each test.
+import 'react-testing-library/cleanup-after-each';
+
+// this adds jest-dom's custom assertions
+import 'jest-dom/extend-expect';
+```
+
+Here's an example of using `react-testing-library` and `jest-dom` for testing that the `<App />` component renders "Welcome to React".
+
+```js
+import React from 'react';
+import { render } from 'react-testing-library';
+import App from './App';
+
+it('renders welcome message', () => {
+  const { getByText } = render(<App />);
+  expect(getByText('Welcome to React')).toBeInTheDOM();
+});
+```
+
+Learn more about the utilities provided by `react-testing-library` to facilitate testing asynchronous interactions as well as selecting form elements from [the `react-testing-library` documentation](https://github.com/kentcdodds/react-testing-library) and [examples](https://codesandbox.io/s/github/kentcdodds/react-testing-library-examples).
+
 ### Using Third Party Assertion Libraries
 
 We recommend that you use `expect()` for assertions and `jest.fn()` for spies. If you are having issues with them please [file those against Jest](https://github.com/facebook/jest/issues/new), and we’ll fix them. We intend to keep making them better for React, supporting, for example, [pretty-printing React elements as JSX](https://github.com/facebook/jest/pull/1566).