Fix indentation of example, and add a protip about ES6 classes.
Fix indentation of example, and add a protip about ES6 classes.

petersendidit/eslint-plugin-react,yannickcr/eslint-plugin-react,lencioni/eslint-plugin-react,mathieumg/eslint-plugin-react,benmosher/eslint-plugin-react,AlexKVal/eslint-plugin-react,pwmckenna/eslint-plugin-react,Khan/eslint-plugin-react