Add Sonar Quality Gate badge. (#47)

Author: rlubke

README.md

@@ -1,6 +1,7 @@
 # Coherence Python Client
 
 ![CI/CD](https://github.com/oracle/coherence-py-client/actions/workflows/validate.yml/badge.svg)
+[![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=oracle_coherence-py-client&metric=alert_status)](https://sonarcloud.io/project/overview?id=oracle_coherence-py-client)
 [![License](http://img.shields.io/badge/license-UPL%201.0-blue.svg)](https://oss.oracle.com/licenses/upl/)
 
 <img src=https://oracle.github.io/coherence/assets/images/logo-red.png width="30%"><img>