add code coverage

Author: gkorland

README.md

@@ -2,6 +2,7 @@
 [![CircleCI](https://circleci.com/gh/RedisGraph/JRedisGraph/tree/master.svg?style=svg)](https://circleci.com/gh/RedisGraph/JRedisGraph/tree/master)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.redislabs/jredisgraph/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.redislabs/jredisgraph)
 [![GitHub issues](https://img.shields.io/github/release/RedisGraph/JRedisGraph.svg)](https://github.com/RedisGraph/JRedisGraph/releases/latest)
+[![Codecov](https://codecov.io/gh/RedisGraph/JRedisGraph/branch/master/graph/badge.svg)](https://codecov.io/gh/RedisGraph/JRedisGraph)
 
 # JRedisGraph
 RedisGraph Java client