update README to add coverage icon

update README to add coverage icon

README.md

@@ -3,6 +3,7 @@
 # Sentinel: Sentinel of Your Application
 
 [![Travis Build Status](https://travis-ci.org/alibaba/Sentinel.svg?branch=master)](https://travis-ci.org/alibaba/Sentinel)
+[![codecov](https://codecov.io/gh/alibaba/Sentinel/branch/master/graph/badge.svg)](https://codecov.io/gh/alibaba/Sentinel)
 [![Maven Central](https://img.shields.io/maven-central/v/com.alibaba.csp/sentinel-core.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:com.alibaba.csp%20AND%20a:sentinel-core)
 [![License](https://img.shields.io/badge/license-Apache%202-4EB1BA.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
 [![Gitter](https://badges.gitter.im/alibaba/Sentinel.svg)](https://gitter.im/alibaba/Sentinel)