Documentation issue: 'gotcha' for new users, staticExtensions in config file must be an array

[akira-kurogane]

If a user writes into the config file the obvious equivalent of --static.extensions html that will be:

  {
    staticExtensions: 'html'
  }

But this will silently fail.

At least for now the syntax must be:

  {
    staticExtensions: [ 'html' ]
  }