add codecov badge (knative#9723)

README.md

@@ -5,6 +5,7 @@
 [![Releases](https://img.shields.io/github/release-pre/knative/serving.svg?sort=semver)](https://github.com/knative/serving/releases)
 [![LICENSE](https://img.shields.io/github/license/knative/serving.svg)](https://github.com/knative/serving/blob/master/LICENSE)
 [![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://knative.slack.com)
+[![codecov](https://codecov.io/gh/knative/serving/branch/master/graph/badge.svg)](https://codecov.io/gh/knative/serving)
 
 Knative Serving builds on Kubernetes to support deploying and serving of
 applications and functions as serverless containers. Serving is easy to get