Add CHANGELOG entry for #deliver_later

actionmailer/CHANGELOG.md

+*   Added #deliver_later in addition to #deliver, which will enqueue a job to render and
+    deliver the mail instead of delivering it right at that moment. The job is enqueued 
+    using the new Active Job framework in Rails, and will use whatever queue is configured for Rails.
+    
+    *DHH/Abdelkader Boudih/Cristian Bica*
+    
+
 *   Make ActionMailer::Previews methods class methods. Previously they were
     instance methods and ActionMailer tries to render a message when they
     are called.