adding information about environment filters to the README
adding information about environment filters to the README

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