Error running start script on Windows at Chapter 6

[talohana]

I'm currently going through chapter 6, working with React.
The start script sets environment variable named EXTEND_ESLINT, which on windows outputs the following error:

'EXTEND_ESLINT' is not recognized as an internal or external command, operable program or batch file.

I ended up using cross-env and updating the start script to:

  "scripts": {
    "start": "cross-env EXTEND_ESLINT=true react-scripts start"
  },

And maybe you could add some explanation of why we need this dependency where you explain other project dependencies.

I would happily submit a PR (with some instructions on how to apply them to all branches).