Add instructions for heroku gem to Heroku addon README
Add instructions for heroku gem to Heroku addon README

airbrake/airbrake,thoughtbot/airbrake,square/hoptoad_notifier,airbrake/airbrake,benjaminoakes/airbrake,nick-desteffen/airbrake,Shopify/airbrake,stevecrozz/hydraulic_brake,thoughtbot/hoptoad_notifier,nick-desteffen/airbrake,CarlosCD/airbrake,CarlosCD/airbrake