Added Autoscaler guide to advanced heroku deployment
Added Autoscaler guide to advanced heroku deployment

This was the proposed change to add an optional step for including and configuring the Autoscaler Gem in the HEROKU.md doc. I've included a pastable version of the Sidekiq initializer file that only adds the autoscaler middleware in production.   
natefinch/discourse,tadp/learnswift,natefinch/discourse,tadp/learnswift,tadp/learnswift