message: Add quit utterance to docs
diff: @@ -768,6 +768,7 @@ Botkit provides several built in variables that are automatically available to a
  |--- |---
  | bot.utterances.yes | Matches phrases like yes, yeah, yup, ok and sure.
  | bot.utterances.no | Matches phrases like no, nah, nope
  +| bot.utterances.quit | Matches phrases like, cancel, exit, stop
  ##### Conversation Control Functions
  