Update docs for nib usage

[optikalefx]

since nib is super popular with stylus, please add the last comment in this thread to the docs for how to use nib with stylus. It's not clear anywhere in here and requires digging.

ember-cli/ember-cli#650

Code from there

npm install --save-dev nib

  var nib = require('nib');
  var app = new EmberApp(defaults, {
    stylusOptions: {
      use: [nib()],
      inputFile: 'style.styl',
      outputFile: 'style.css'
    }
  }

[optikalefx]

Should also add that you need to @import 'nib' in your app.styl file.

[drewcovi]

very cool stuff. ive always used ember-cli-autoprefixer, but ill try to get that updated here within the week.