SortableContainer can't wrap a Fragment

Encountered an issue yesterday trying to wrap a component with SortableContainer HOC. Despite everything appearing as I would expect through the console and React Dev Tools, dragging functionality refused to work.

After a few hours debugging, I found a clue to my problem in the react-sortable-HOC Gitter channel. Hat-tip to @nuthinking for this line:

`"Just realised that, obviously, a SortableContanier can't be a Fragment :D"`

In retrospect I see why, but I can't help but feel there will be others in the future who make the same mistake I did.

Possible to add a warning in the FAQ or documentation? I'm happy to whip up a PR for the change.

_To clarify: I'm not asking for SortableContainer to work with Fragments, just increasing visibility for others who end up as stumped as I did._

Thanks! Fantastic package :)

Review the repository's FAQ and documentation entry points for guidance about SortableContainer and React Fragments. Add a clear warning that SortableContainer cannot wrap a Fragment, and consider the work complete when the limitation is visible where users look for usage guidance.

Written by the indexing model from the issue text.