Need update docs for set-data

[thaycacac]

First check https://github.com/SortableJS/Vue.Draggable/blob/master/CONTRIBUTING.md

Actual Solution

I see :set-data="setData" when read project vue-element-admin. But i don't see it document

Expected Solution

Update docs how to use set-data

[David-Desmaisons]

setData is part of https://github.com/SortableJS/Sortable API and documented there.

[heyakyra]

Sortable documents direct method access setData on the Sortable instance, but Vue.Draggable users need documentation on how to use it through Vue components' :set-data property.

Related: #963