Document default_options configuration in ActionMailer guide [ci skip]

guides/source/action_mailer_basics.textile

@@ -458,6 +458,7 @@ The following configuration options are best made in one of the environment file
 |+perform_deliveries+|Determines whether deliveries are actually carried out when the +deliver+ method is invoked on the Mail message. By default they are, but this can be turned off to help functional testing.|
 |+deliveries+|Keeps an array of all the emails sent out through the Action Mailer with delivery_method :test. Most useful for unit and functional testing.|
 |+async+|Setting this flag will turn on asynchronous message sending, message rendering and delivery will be pushed to <tt>Rails.queue</tt> for processing.|
+|+default_options+|Allows you to set default values for the <tt>mail</tt> method options (<tt>:from</tt>, <tt>:reply_to</tt>, etc.).|
 
 h4. Example Action Mailer Configuration
 
@@ -472,6 +473,7 @@ config.action_mailer.delivery_method = :sendmail
 # }
 config.action_mailer.perform_deliveries = true
 config.action_mailer.raise_delivery_errors = true
+config.action_mailer.default_options = {from: "no-replay@example.org"}
 </ruby>
 
 h4. Action Mailer Configuration for GMail