Document how delivery works

lib/bugsnag/delivery.rb

@@ -1,10 +1,19 @@
 module Bugsnag
   module Delivery
     class << self
+      # Add a delivery method to the list of supported methods. Any registered
+      # method can then be used by name in Configuration.
+      #
+      # require 'bugsnag'
+      # Bugsnag::Delivery.register(:my_delivery_queue, MyDeliveryQueue)
+      # Bugsnag.configure do |config|
+      #   config.delivery_method = :my_delivery_queue
+      # end
       def register(name, delivery_method)
         delivery_methods[name.to_sym] = delivery_method
       end
 
+      # Reference a delivery method by name
       def [](name)
         delivery_methods[name.to_sym]
       end