Adding Go Report Card badge

README.md

 <a href="https://caddyserver.com"><img src="https://caddyserver.com/resources/images/caddy-lower.png" alt="Caddy" width="350"></a>
 
 [![community](https://img.shields.io/badge/community-forum-ff69b4.svg?style=flat-square)](https://forum.caddyserver.com) [![twitter](https://img.shields.io/badge/twitter-@caddyserver-55acee.svg?style=flat-square)](https://twitter.com/caddyserver) [![Documentation](https://img.shields.io/badge/godoc-reference-blue.svg?style=flat-square)](https://godoc.org/github.com/mholt/caddy) [![Linux Build Status](https://img.shields.io/travis/mholt/caddy.svg?style=flat-square&label=linux+build)](https://travis-ci.org/mholt/caddy) [![Windows Build Status](https://img.shields.io/appveyor/ci/mholt/caddy.svg?style=flat-square&label=windows+build)](https://ci.appveyor.com/project/mholt/caddy)
+[![Go Report Card](https://goreportcard.com/badge/github.com/mholt/caddy?style=flat-square)](https://goreportcard.com/report/mholt/caddy)
+
 
 Caddy is a general-purpose web server for Windows, Mac, Linux, BSD, and
 [Android](https://github.com/mholt/caddy/wiki/Running-Caddy-on-Android). It is