message: Add google hangouts chat doc link to the readme file
diff: @@ -16,6 +16,7 @@ All documentation for Botkit now lives at our official documentation site, [http
  * [Twilio SMS](https://botkit.ai/docs/readme-twiliosms.html)
  * [Twilio IPM](https://botkit.ai/docs/readme-twilioipm.html)
  * [Microsoft Bot Framework](https://botkit.ai/docs/readme-botframework.html)
  + * [Google Hangouts Chat](https://botkit.ai/docs/google-hangouts-chat.html)
  * [Extending Botkit with Plugins and Middleware](https://botkit.ai/docs/middleware.html)
  * [Message Pipeline](https://botkit.ai/docs/readme-pipeline.html)
  * [List of current plugins](https://botkit.ai/docs/readme-middlewares.html)
  