Update config.md

[Ollynov]

The following is not the first configuration step:

http: {
    root: 'http://localhost:3000/',
    headers: {
      Authorization: 'Basic YXBpOnBhc3N3b3Jk'
    }
  },

The first step is missing. Here are at least 50 people who wondered the same:
vuejs-templates/webpack#424