Update API.md

Added onStateChange doc

docs/API.md

@@ -21,6 +21,7 @@
 | `sceneStyle`     | `Style` |  | Style applied to all scenes (optional) |
 | `backAndroidHandler`     | `Function` |  | Allows custom control of hardwareBackPress in Android (optional). For more info check [BackHandler](https://facebook.github.io/react-native/docs/backhandler.html).  |
 | `uriPrefix`     | `string` |  | A uri prefix to strip from incoming urls for deep linking. For example, if you wanted to support deep linking from `www.example.com/user/1234/`, then you could pass `example.com` to only match paths against `/user/1234/`. |
+| `onStateChange`   | `Function` |  | function to be called every time when navigation state is changed. Useful for mobx stores to change observables (set scene to `Actions.currentScene`, etc.) |
 
 ## Scene:
 The basic routing component for this router, all `<Scene>` components require a `key` prop that must be unique. A parent `<Scene>` must have a `component` as a `prop` as it will act as a grouping component for its children.