Updates the README page to document this update

Author: Jesús Burgos

README.md

@@ -52,10 +52,6 @@ end
 Every setting is handled by the class `RailsSettings::SettingObject`. You can use your own class, e.g. for validations:
 
 ```ruby
-class Project < ActiveRecord::Base
-  has_settings :info, :class_name => 'ProjectSettingObject'
-end
-
 class ProjectSettingObject < RailsSettings::SettingObject
   validate do
     unless self.owner_name.present? && self.owner_name.is_a?(String)
@@ -65,6 +61,25 @@ class ProjectSettingObject < RailsSettings::SettingObject
 end
 ```
 
+Then you can use it like this:
+
+```ruby
+class Project < ActiveRecord::Base
+  has_settings :info, :class_name => 'ProjectSettingObject'
+end
+```
+
+Or use it only on some of the settings:
+
+```ruby
+class Project < ActiveRecord::Base
+  has_settings do |s|
+    s.key :calendar # Will use the default RailsSettings::SettingObject
+    s.key :info, :class_name => 'ProjectSettingObject'
+  end
+end
+```
+
 ### Set settings
 
 ```ruby