[common] add codecov badge

Author: pbalcer

README.md

@@ -4,6 +4,8 @@
 [![CodeQL](https://github.com/oneapi-src/unified-runtime/actions/workflows/codeql.yml/badge.svg)](https://github.com/oneapi-src/unified-runtime/actions/workflows/codeql.yml)
 [![Bandit](https://github.com/oneapi-src/unified-runtime/actions/workflows/bandit.yml/badge.svg)](https://github.com/oneapi-src/unified-runtime/actions/workflows/bandit.yml)
 [![Coverity](https://scan.coverity.com/projects/28213/badge.svg)](https://scan.coverity.com/projects/oneapi-src-unified-runtime)
+[![codecov.io](https://codecov.io/github/oneapi-src/unified-runtime/coverage.svg?branch=main)]
+(https://codecov.io/github/oneapi-src/unified-runtime?branch=master)
 
 <!-- TODO: add general description and purpose of the project -->