ESLint instructions should include `eslint --init`

[shepmaster]

Following the steps in the book:

1. npm i eslint --save-dev
2. Add "test:lint": "eslint . --ext .js --ext .jsx --cache" to package.json
3. npm run test:lint

You will get the error:

Oops! Something went wrong! :(

ESLint couldn't find a configuration file. To set up a configuration file for this project, please run:

    eslint --init

The error message is straightforward to fix, but it may be worth including it in the text.

[bebraw]

Thanks!