document Droppable's direction prop (#151)

README.md

@@ -476,6 +476,7 @@ import { Droppable } from 'react-beautiful-dnd';
 - `droppableId`: A *required* `DroppableId(string)` that uniquely identifies the droppable for the application. Please do not change this prop - especially during a drag.
 - `type`: An *optional* `TypeId(string)` that can be used to simply accept a class of `Draggable`. For example, if you use the type `PERSON` then it will only allow `Draggable`s of type `PERSON` to be dropped on itself. `Draggable`s of type `TASK` would not be able to be dropped on a `Droppable` with type `PERSON`. If no `type` is provided, it will be set to `'DEFAULT'`. Currently the `type` of the `Draggable`s within a `Droppable` **must be** the same. This restriction might be loosened in the future if there is a valid use case.
 - `isDropDisabled`: An *optional* flag to control whether or not dropping is currently allowed on the `Droppable`. You can use this to implement your own conditional dropping logic. It will default to `false`.
+- `direction`: The direction in which items flow in this droppable. Options are `vertical` (default) and `horizontal`.
 
 ### Children function