Added GoReportCard to README

This patch adds the GoReportCard badge (A+) to the README file.

README.md

@@ -1,4 +1,4 @@
-# REX-Ray [![Build Status](http://travis-ci.org/emccode/rexray.svg?branch=master)](https://travis-ci.org/emccode/rexray) [![Coverage Status](http://coveralls.io/repos/emccode/rexray/badge.svg?branch=master&service=github)](https://coveralls.io/github/emccode/rexray?branch=master) [ ![Download](http://api.bintray.com/packages/emccode/rexray/stable/images/download.svg) ](https://dl.bintray.com/emccode/rexray/stable/latest/)
+# REX-Ray [![Build Status](http://travis-ci.org/emccode/rexray.svg?branch=master)](https://travis-ci.org/emccode/rexray) [![Go Report Card](http://goreportcard.com/badge/emccode/rexray)](http://goreportcard.com/report/emccode/rexray) [![Coverage Status](http://coveralls.io/repos/emccode/rexray/badge.svg?branch=master&service=github)](https://coveralls.io/github/emccode/rexray?branch=master) [ ![Download](http://api.bintray.com/packages/emccode/rexray/stable/images/download.svg) ](https://dl.bintray.com/emccode/rexray/stable/latest/)
 ```REX-Ray``` provides visibility and management of external/underlying storage via guest storage introspection. Available as a Go package, CLI tool, and Linux service, and with built-in third-party support for tools such as ```Docker```, ```REX-Ray``` is easily integrated into any workflow. For example, here's how to list storage for a guest hosted on Amazon Web Services (AWS) with ```REX-Ray```:
 
 ```bash