message: info on webhook subscriptions for handover events
diff: @@ -93,10 +93,10 @@ Normal messages will be sent to your bot using the `message_received` event. In
  | facebook_optin | a user has clicked the [Send-to-Messenger plugin](https://developers.facebook.com/docs/messenger-platform/implementation#send_to_messenger_plugin)
  | facebook_referral | a user has clicked on a [m.me URL with a referral param](https://developers.facebook.com/docs/messenger-platform/referral-params)
  | facebook_app_roles | This callback will occur when a page admin changes the role of your application.
  -| standby | This callback will occur when a message has been sent to your page, but your application is not the current thread owner.
  -| facebook_receive_thread_control | This callback will occur when thread ownership for a user has been passed to your application.
  -| facebook_lose_thread_control | This callback will occur when thread ownership for a user has been taken away from your application.
  -| facebook_request_thread_control | This callback will occur when thread ownership is requested by another receiver.
  +| standby | This callback will occur when a message has been sent to your page, but your application is not the current thread owner. (Subscribe to the `standby` webhook to receive this event.)
  +| facebook_receive_thread_control | This callback will occur when thread ownership for a user has been passed to your application. (Subscribe to the `messaging_handovers` webhook to receive this event.)
  +| facebook_lose_thread_control | This callback will occur when thread ownership for a user has been taken away from your application. (Subscribe to the `messaging_handovers` webhook to receive this event.)
  +| facebook_request_thread_control | This callback will occur when thread ownership is requested by another receiver. (Subscribe to the `messaging_handovers` webhook to receive this event.)
  All incoming events will contain the fields `user` and `channel`, both of which represent the Facebook user's ID, and a `timestamp` field.
  