message: m doc, add introduce for listening custom event
diff: - [getSelectedIndex](api-specific.md#getselectedindex)
  - [finishPullDown](api-specific.md#finishpulldown)
  - [finishPullUp](api-specific.md#finishpullup)
  + - [on](api-specific.md#on)
  + - [once](api-specific.md#once)
  + - [off](api-specific.md#off)
  - [Events](events.md)
  - [beforeScrollStart](events.md#beforescrollstart)
  - [scrollStart](events.md#scrollstart)
  