docs(readme): adding codecov badge (#249)

Author: ultimateboy

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://ci.deis.io/buildStatus/icon?job=Deis/workflow-cli/master)](https://ci.deis.io/job/Deis/job/workflow-cli/job/master/)
 [![Go Report Card](https://goreportcard.com/badge/github.com/deis/workflow-cli)](https://goreportcard.com/report/github.com/deis/workflow-cli)
 [![codebeat badge](https://codebeat.co/badges/05d314a8-ca61-4211-b69e-e7a3033662c8)](https://codebeat.co/projects/github-com-deis-workflow-cli)
+[![codecov](https://codecov.io/gh/deis/workflow-cli/branch/master/graph/badge.svg)](https://codecov.io/gh/deis/workflow-cli)
 
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