chore(README.md): add codecov icon

Author: duanhongyi

README.md

@@ -1,6 +1,7 @@
 # Drycc Pkg
 
 [![Build Status](https://travis-ci.org/drycc/pkg.svg?branch=master)](https://travis-ci.org/drycc/pkg)
+[![codecov](https://codecov.io/gh/drycc/pkg/branch/master/graph/badge.svg)](https://codecov.io/gh/drycc/pkg)
 [![Go Report Card](https://goreportcard.com/badge/github.com/drycc/pkg)](https://goreportcard.com/report/github.com/drycc/pkg)
 [![GoDoc](https://godoc.org/github.com/drycc/pkg?status.svg)](https://godoc.org/github.com/drycc/pkg)