doc: Update README.md and add QualityGate code maintainability badge (#2144)

README.md

@@ -7,6 +7,7 @@
 [![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)
+[![Maintainability](https://cloud.quality-gate.com/dashboard/api/badge?projectName=alibaba_Sentinel&branchName=master)](https://cloud.quality-gate.com/dashboard/branches/7825#overview)
 
 ## Introduction