Update marker.md to include the draggable prop. (#902)

Author: anami

docs/marker.md

@@ -16,6 +16,7 @@
 | `flat` | `Boolean` |  | Sets whether this marker should be flat against the map true or a billboard facing the camera false.
 | `identifier` | `String` |  | An identifier used to reference this marker at a later date.
 | `rotation` | `Float` |  | A float number indicating marker's rotation angle.
+| `draggable` | `<null>` |  | This is a non-value based prop. Adding this allows the marker to be draggable (re-positioned).
 
 ## Events