📚doc: Add a badge that notices coverage status

Author: negokaz

README.md

@@ -2,6 +2,7 @@ akka-entity-replication
 ===
 
 [![CI](https://github.com/lerna-stack/akka-entity-replication/workflows/CI/badge.svg?branch=master)](https://github.com/lerna-stack/akka-entity-replication/actions?query=workflow%3ACI+branch%3Amaster)
+[![Coverage Status](https://coveralls.io/repos/github/lerna-stack/akka-entity-replication/badge.svg)](https://coveralls.io/github/lerna-stack/akka-entity-replication)
 
 ## Introduction