Add comment about why `image/x-icon` is used

Ref: https://github.com/h5bp/html5-boilerplate/commit/37b5fec090d00f38de64b591bcddcb205aadf8ee
     http://blogs.msdn.com/b/ieinternals/archive/2011/02/11/ie9-release-candidate-minor-changes-list.aspx