Improve README.md

[marcelometal]

Added syntax highlight

Before submitting a pull request, please make sure the following is done...

1. Fork the repo and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure that:

• The test suite passes (npm test)
• Your code lints (npm run lint) and passes Flow (npm run flow)
• You have followed the testing guidelines

5. If you haven't already, complete the CLA.

Please use the simple form below as a guideline for describing your pull request.

Thanks for contributing to Draft.js!

Summary

[...]

Test Plan

[...]

[davidchang]

thanks @marcelometal