Add badge for code coverage

Adding a badge for code coverage

README.md

@@ -1,6 +1,7 @@
 # Microsoft Azure Container Networking
 
 [![CircleCI](https://circleci.com/gh/Azure/azure-container-networking/tree/master.svg?style=svg)](https://circleci.com/gh/Azure/azure-container-networking/tree/master) [![Go Report Card](https://goreportcard.com/badge/github.com/Azure/azure-container-networking)](https://goreportcard.com/report/github.com/Azure/azure-container-networking) ![GitHub release](https://img.shields.io/github/release/Azure/azure-container-networking.svg)
+[![codecov](https://codecov.io/gh/Azure/azure-container-networking/branch/master/graph/badge.svg)](https://codecov.io/gh/Azure/azure-container-networking)
 
 ## Overview
 This repository contains container networking plugins for Linux and Windows containers running on Azure: