Creating a draggable container without using findDOMNode()

[seiji619]

Creating a dragula object without using findDOMNode() is not possible right now, if the container element is a custom React component. I understand Facebook will eventually deprecate this method, so it is an important feature to have.

[iamdanthedev]

		this.draggable = Dragula({
			isContainer: el => el.classList.contains('drag-container'),
		})

[jondoesntgit]

Now that findDOMNode is deprecated, the README should be updated.