docs: update README.md (#171)

Add `import React from 'react'` to simple debug example.

<!--
Thanks for your interest in the project. Bugs filed and PRs submitted are appreciated!

Please make sure that you are familiar with and follow the Code of Conduct for
this project (found in the CODE_OF_CONDUCT.md file).

Also, please make sure you're familiar with and follow the instructions in the
contributing guidelines (found in the CONTRIBUTING.md file).

If you're new to contributing to open source projects, you might find this free
video course helpful: http://kcd.im/pull-request

Please fill out the information below to expedite the review and (hopefully)
merge of your pull request!
-->

<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**:

The example in README.md has a ReferenceError: `React is not defined`

**Why**:

Help others run the example by copying and pasting code.

**How**:

Quick markdown change.

**Checklist**:

- [ ] Documentation N/A
- [ ] TestsN/A
- [x] Ready to be merged
      <!-- In your opinion, is this ready to be merged as soon as it's reviewed? -->
- [ ] Added myself to contributors table
      <!-- this is optional, see the contributing guidelines for instructions -->

Author: arturoromeroslc

README.md

@@ -348,6 +348,7 @@ renders it's HTML directly in the body.
 This method is a shortcut for `console.log(prettyDOM(baseElement))`.
 
 ```javascript
+import React from 'react'
 import {render} from 'react-testing-library'
 
 const HelloWorld = () => <h1>Hello World</h1>