message: Improve docs : How to use fb insights api
diff: @@ -21,6 +21,7 @@ Table of Contents
  * [Silent and No Notifications](#silent-and-no-notifications)
  * [Messenger code API](#messenger-code-api)
  * [Attachment upload API](#attachment-upload-api)
  +* [Messaging Insights API](#messaging-insights-api)
  * [Running Botkit with an Express server](#use-botkit-for-facebook-messenger-with-an-express-web-server)
  ## Getting Started
  @@ -570,6 +571,24 @@ var taggedMessage = {
  bot.reply(message, taggedMessage);
  ```
  +## Messaging Insights API
  +
  +The Messaging Insights API, allows you to programatically retrieve the same information that appears in the Page Insights tab of your Facebook Page.
  +
  +To get insights with Botkit, you shuld call ```controller.api.insights.get_insights(...)``` with parameters metrics, since and until :
  +
  +| Parameter | Description
  +|--- |---
  +| metrics | An array or a comma-separated list of metrics to return.
  +| since | UNIX timestamp of the start time to get the metric for.
  +| until | UNIX timestamp of the end time to get the metric for.
  +
  +```javascript
  +controller.api.insights.get_insights('page_messages_active_threads_unique', null, null, function (err, body) {
  + console.log(body);
  + console.log(err);
  +});
  +```
  ## Use BotKit for Facebook Messenger with an Express web server
  Instead of the web server generated with setupWebserver(), it is possible to use a different web server to receive webhooks, as well as serving web pages.
  