clarified that this widget doesn't support sublayers

It's confusing that the description and readme indicate that this widget supports sublayers, when it does not (see #6).

If you merge this, you should also remove " and sublayers" from the repo description.

On a side note, is support for sublayers on the roadmap? If not, would you be open to a PR that supports at least one case (dynamic map service `visibleLayers`)?