[docs] Adding a section about 'url_with_processing.'

README.textile

@@ -102,6 +102,35 @@ being processed.
 
 </code></pre>
 
+h4. Have processing? status available, but construct image URLs as if delayed_paperclip wasn't present
+
+If you define the #{attachment_name}_processing column, but set the url_with_processing option to false,
+this opens up other options (other than modifying the url that paperclip returns) for giving feedback to
+the user while the image is processing.  This is useful for advanced situations, for example when dealing
+with caching systems.
+
+Note especially the method .processing? which passes through the value of the boolean created via migration.
+
+<pre><code>
+
+  class User < ActiveRecord::Base
+    has_attached_file :avatar
+
+    process_in_background :avatar, :url_with_processing => false
+  end
+
+  @user = User.new(:avatar => File.new(...))
+  @user.save
+  @user.avatar.url #=> "/system/images/3/original/IMG_2772.JPG?1267562148"
+  @user.avatar.processing? #=> true
+  Delayed::Worker.new.work_off
+
+  @user.reload
+  @user.avatar.url #=> "/system/images/3/original/IMG_2772.JPG?1267562148"
+  @user.avatar.processing? #=> false
+
+</code></pre>
+
 h4. Only process certain styles
 
 This is useful if you don't want the background job to reprocess all styles.