message: provide definition of RTM
  The readme makes numerous references to "RTM" but never explains that this stands for Slack's Real Time Messaging API. This pull request adds that reference, including a link to Slack's Real Time Messaging API documentation.
diff: @@ -73,7 +73,7 @@ This is particularly true if you store and use API tokens on behalf of users oth
  The Botkit Slack controller object can be configured in a few different ways, depending on the type of integration you are building.
  -A simple single-team bot that uses the RTM api can be instantated without any special options:
  +A simple single-team bot that uses Slack's [Real Time Messaging (RTM) API](https://api.slack.com/rtm) can be instantiated without any special options:
  ```javascript
  var controller = Botkit.slackbot({});
  