Update README with go report card

Author: marshyski

README.md

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 A GO API for managing yum repos and  NGINX to serve them
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-[![CircleCI](https://circleci.com/gh/FINRAOS/yum-nginx-api/tree/master.svg?style=svg)](https://circleci.com/gh/FINRAOS/yum-nginx-api/tree/master)
+[![CircleCI](https://circleci.com/gh/FINRAOS/yum-nginx-api/tree/master.svg?style=svg)](https://circleci.com/gh/FINRAOS/yum-nginx-api/tree/master) [![Go Report Card](https://goreportcard.com/badge/github.com/FINRAOS/yum-nginx-api)](https://goreportcard.com/report/github.com/FINRAOS/yum-nginx-api)
 
 [yum-nginx-api][1] is a go API for uploading RPMs to yum repositories and also configurations for running NGINX to serve them.