message: docs(readme): add onPaneContextMenu
diff: @@ -96,6 +96,7 @@ const BasicFlow = () => <ReactFlow elements={elements} />;
  - `onMoveEnd()`: called when user ends panning or zooming
  - `onSelectionChange(elements: Elements)`: called when user selects one or multiple elements
  - `onPaneClick(evt: MouseEvent)`: called when user clicks directly on the canvas
  +- `onPaneContextMenu(evt: MouseEvent)`: called when user does a right-click on the canvas
  #### Interaction
  - `nodesDraggable`: default: `true`. This applies to all nodes. You can also change the behavior of a specific node with the `draggable` node option.
  