add codecov badge

Author: trajinthan

README.md

@@ -5,6 +5,7 @@ Ballerina Regex Library
   [![GitHub Last Commit](https://img.shields.io/github/last-commit/ballerina-platform/module-ballerina-regex.svg)](https://github.com/ballerina-platform/module-ballerina-regex/commits/main)
   [![Github issues](https://img.shields.io/github/issues/ballerina-platform/ballerina-standard-library/module/regex.svg?label=Open%20Issues)](https://github.com/ballerina-platform/ballerina-standard-library/labels/module%2Fregex)
   [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
+  [![codecov](https://codecov.io/gh/ballerina-platform/module-ballerina-regex/branch/main/graph/badge.svg)](https://codecov.io/gh/ballerina-platform/module-ballerina-regex)
 
 This Regex library is one of the standard libraries of the <a target="_blank" href="https://ballerina.io/">Ballerina</a> language.