Update README

Notice about dependent option

Author: LTe

README.md

@@ -41,7 +41,8 @@ Usage
 class User < ActiveRecord::Base
   acts_as_messageable :table_name => "table_with_messages", # default 'messages'
                       :required   => :body                  # default [:topic, :body]
-                      :class_name => "CustomMessages"       # default "ActsAsMessageable::Message"
+                      :class_name => "CustomMessages"       # default "ActsAsMessageable::Message",
+                      :dependent  => :destroy               # default :nullify
 end
 ```