Add gem badge

Author: William Tsai

README.md

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 # Error Response
 
+[![Gem Version](https://badge.fury.io/rb/error_response.svg?icon=si%3Arubygems)](https://badge.fury.io/rb/error_response)
+
 `Error Response` is a json response gem to help you easily manage all your custom error statuses in your Rails application.
 
 ## Installation