Update README.md (#22)

Author: feng-tao

README.md

@@ -1,6 +1,7 @@
 # Amundsen Search service
 [![PyPI version](https://badge.fury.io/py/amundsen-search.svg)](https://badge.fury.io/py/amundsen-search)
 [![Build Status](https://api.travis-ci.com/lyft/amundsensearchlibrary.svg?branch=master)](https://travis-ci.com/lyft/amundsensearchlibrary)
+[![Coverage Status](https://img.shields.io/codecov/c/github/lyft/amundsensearchlibrary/master.svg)](https://codecov.io/github/lyft/amundsensearchlibrary?branch=master)
 [![License](http://img.shields.io/:license-Apache%202-blue.svg)](LICENSE)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)
 [![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://bit.ly/2FVq37z)