Add codecov.io badge to README

Add codecov.io based coverage report badge to README.

README.md

@@ -3,6 +3,7 @@
 [![License](https://img.shields.io/github/license/gonvenience/bunt.svg)](https://github.com/gonvenience/bunt/blob/master/LICENSE)
 [![Go Report Card](https://goreportcard.com/badge/github.com/gonvenience/bunt)](https://goreportcard.com/report/github.com/gonvenience/bunt)
 [![Build Status](https://travis-ci.org/gonvenience/bunt.svg?branch=master)](https://travis-ci.org/gonvenience/bunt)
+[![Codecov](https://img.shields.io/codecov/c/github/gonvenience/bunt/master.svg)](https://codecov.io/gh/gonvenience/bunt)
 [![GoDoc](https://godoc.org/github.com/gonvenience/bunt/pkg?status.svg)](https://godoc.org/github.com/gonvenience/bunt)
 [![Release](https://img.shields.io/github/release/gonvenience/bunt.svg)](https://github.com/gonvenience/bunt/releases/latest)