update README

README.md

@@ -12,14 +12,14 @@ example site: http://stark-cloud-4321.herokuapp.com/
 
 1. add following line to `Gemfile`
 
-```
+```ruby
 gem 'paperclip'
 gem 'paperclip-qiniu'
 ```
 
 2. edit your `config/application.rb`
 
-```
+```ruby
 module PaperclipQiniuExample
   class Application < Rails::Application
     # ....
@@ -29,18 +29,21 @@ module PaperclipQiniuExample
         :secret_key => ENV['QINIU_SECRET_KEY'] || raise("set env QINIU_SECRET_KEY")
       },
       :bucket => "paperclip-qiniu-example",
-      :use_timestamp => false
+      :use_timestamp => false,         # required, and must be set to false
+      :qiniu_host => "cdn.example.com" # optional
     }
   end
 end
 ```
 
+for more information on `qiniu_host`, read http://docs.qiniutek.com/v2/sdk/ruby/#publish
+
 3. add a model like this
 
-```
+```ruby
 class Image < ActiveRecord::Base
   attr_accessible :file
-  has_attached_file :file, :styles => { :medium => "300x300>", :thumb => "100x100>" }
+  has_attached_file :file, :styles => { :medium => "300x300>", :thumb => "100x100>" }, :path => ":class/:attachment/:id/:style/:basename.:extension"
   validates :file, :attachment_presence => true
 end
 ```