Include link to Airbrake.notify section in airbrake-ruby
Include link to Airbrake.notify section in airbrake-ruby

We mention preferring `notify_airbrake` to `Airbrake.notify` without
first saying what `Airbrake.notify` is. This adds a link to the
relevant section in the airbrake-ruby repo.

airbrake/airbrake,airbrake/airbrake,nick-desteffen/airbrake,nick-desteffen/airbrake