Add coverage badge

Author: dmaccormack

README.md

@@ -4,6 +4,7 @@
 [![javadoc](https://javadoc.io/badge2/org.jsonurl/jsonurl-core/javadoc.svg)](https://javadoc.io/doc/org.jsonurl/jsonurl-core)
 [![Build Status](https://travis-ci.com/jsonurl/jsonurl-java.svg?branch=master)](https://travis-ci.com/jsonurl/jsonurl-java)
 [![Quality Gate](https://sonarcloud.io/api/project_badges/measure?project=jsonurl_jsonurl-java&metric=alert_status)](https://sonarcloud.io/dashboard?id=jsonurl_jsonurl-java)
+[![Coverage](https://sonarcloud.io/api/project_badges/measure?project=jsonurl_jsonurl-java&metric=coverage)](https://sonarcloud.io/dashboard?id=jsonurl_jsonurl-java)
 [![Lines of Code](https://sonarcloud.io/api/project_badges/measure?project=jsonurl_jsonurl-java&metric=ncloc)](https://sonarcloud.io/dashboard?id=jsonurl_jsonurl-java)
 [![Security Rating](https://sonarcloud.io/api/project_badges/measure?project=jsonurl_jsonurl-java&metric=security_rating)](https://sonarcloud.io/dashboard?id=jsonurl_jsonurl-java)
 [![Vulnerabilities](https://sonarcloud.io/api/project_badges/measure?project=jsonurl_jsonurl-java&metric=vulnerabilities)](https://sonarcloud.io/dashboard?id=jsonurl_jsonurl-java)