Added sample migration to the README

git-svn-id: https://svn.thoughtbot.com/plugins/paperclip/trunk@455 7bbfaf0e-4d1d-0410-9690-a8bb5f8ef2aa

README

@@ -12,6 +12,22 @@ In your model:
     has_attached_file :avatar, :styles => { :medium => "300x300>", :thumb => "100x100>" }
   end
 
+In your migrations:
+
+  class AddAvatarColumsToUser < ActiveRecord::Migration
+    def self.up
+      add_column :users, :avatar_file_name, :string
+      add_column :users, :avatar_content_type, :string
+      add_column :users, :avatar_file_size, :integer
+    end
+
+    def self.down
+      remove_column :users, :avatar_file_name
+      remove_column :users, :avatar_content_type
+      remove_column :users, :avatar_file_size
+    end
+  end
+
 In your edit and new views:
 
   <% form_for :user, @user, :url => user_path, :html => { :multipart => true } do |form| %>