update README

README.md

@@ -40,6 +40,7 @@ class Image < ActiveRecord::Base
   attr_accessible :file
   has_attached_file :file, :path => ":class/:attachment/:id/:basename.:extension"
   validates :file, :attachment_presence => true
+  validates_attachment_content_type :file, :content_type => /\Aimage\/.*\Z/
 end
 ```