README.md in compare_linker-1.2.0 vs README.md in compare_linker-1.2.1

- old
+ new
@@ -4,9 +4,22 @@
 
 ![screen shot 2014-01-27 at 2 25 14 am](https://f.cloud.github.com/assets/10515/2004469/de374152-86ae-11e3-84a0-19e2ef40b959.png)
 
 [GitHub Compare View](https://github.com/blog/612-introducing-github-compare-view) rocks.But [diff of Gemfile.lock](https://github.com/kyanny/compare_linker_demo/pull/14/files) sucks. So I made Compare Linker.
 
+## Usage
+
+```ruby
+require 'compare_linker'
+
+ENV['OCTOKIT_ACCESS_TOKEN'] = 'xxx'
+
+compare_linker = CompareLinker.new('masutaka/compare_linker', '17')
+compare_linker.formatter = CompareLinker::Formatter::Markdown.new
+comment = compare_linker.make_compare_links.to_a.join("\n")
+compare_linker.add_comment('masutaka/compare_linker', '17', comment)
+```
+
 ## Rack app for listening GitHub Webhook
 
 There's rack application for Compare Linker with GitHub's Webhook.
 
 https://github.com/kyanny/compare_linker_rack_app