message: docs(readme): add custom node props section
diff: @@ -128,6 +128,17 @@ You could now use the type `special` for a node.
  The `default`, `input` and `output` types would be still available except you overwrote one of them.
  There is an example of a custom node implementation in the [custom node example](/example/src/CustomNode).
  +## Custom Node Props
  +
  +Your custom nodes are wrapped so that the basic functions like dragging or selecting work. Custom nodes receive the following props:
  +
  +- `id`: string
  +- `data`: object
  +- `type`: string
  +- `selected`: boolean
  +- `sourcePosition`: string
  +- `targetPosition`: string
  +
  ## Handle Component
  We export a `Handle` component as a helper for your custom nodes:
  