add go report card to readme

README.md

@@ -3,6 +3,7 @@
 <img align="right" src="https://raw.githubusercontent.com/gin-gonic/gin/master/logo.jpg">
 [![Build Status](https://travis-ci.org/gin-gonic/gin.svg)](https://travis-ci.org/gin-gonic/gin)
 [![Coverage Status](https://coveralls.io/repos/gin-gonic/gin/badge.svg?branch=master)](https://coveralls.io/r/gin-gonic/gin?branch=master)
+[![Go Report Card](https://goreportcard.com/badge/github.com/gin-gonic/gin)]
 [![GoDoc](https://godoc.org/github.com/gin-gonic/gin?status.svg)](https://godoc.org/github.com/gin-gonic/gin)
 [![Join the chat at https://gitter.im/gin-gonic/gin](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/gin-gonic/gin?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)