message: add reference to message_echo event in docs
diff: @@ -80,6 +80,7 @@ Normal messages will be sent to your bot using the `message_received` event. In
  | message_received | a message was received by the bot
  | facebook_postback | user clicked a button in an attachment and triggered a webhook postback
  | message_delivered | a confirmation from Facebook that a message has been received
  +| message_echo | if enabled in Facebook, an "echo" of any message sent by the bot
  | message_read | a confirmation from Facebook that a message has been read
  | facebook_account_linking | a user has started the account linking
  | facebook_optin | a user has clicked the [Send-to-Messenger plugin](https://developers.facebook.com/docs/messenger-platform/implementation#send_to_messenger_plugin)
  