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. It includes a pastable version of the Sidekiq initializer file that only adds the autoscaler middleware in production. 

Although it's probably preferable not to have users editing the Sidekiq init file, the only other option as far as I can see would be to add Heroku specific filters and settings. Probably a bad rabbit hole to go down, especially since there will no doubt be other suitable hosts documented in the future such as red Hat's Openshift (If they ever get round to updating postgres).