Add addListener to navigation prop docs

Author: brentvatne

docs/navigation-prop.md

@@ -9,6 +9,7 @@ Each `screen` component in your app is provided with the `navigation` prop autom
 * `this.props.navigation`
   * `navigate` - go to another screen, figures out the action it needs to take to do it
   * `goBack` - close active screen and move back in the stack
+  * `addListener` - subscribe to updates to navigation lifecycle
   * `state` - current state/routes
   * `setParams` - make changes to route's params
   * `dispatch` - send an action to router
@@ -98,6 +99,15 @@ Then you need to supply a key to goBack _FROM_:
 navigation.goBack(SCREEN_KEY_B) // will go to screen A FROM screen B
 ```
 
+### `addListener` - Subscribe to updates to navigation lifecycle
+
+React Navigation emits events to screen components that subscribe to them:
+
+- `willBlur` - the screen will be unfocused
+- `willFocus` - the screen will focus
+- `didFocus` - the screen focused (if there was a transition, the transition completed)
+- `didBlur` - the screen unfocused (if there was a transition, the transition completed)
+
 ### `state` - The screen's current state/route
 
 A screen has access to its route via `this.props.navigation.state`. Each will return an object with the following: