add coverage badge (does not include integration test coverage yet)

Author: hjacobs

README.md

@@ -3,6 +3,8 @@ SProcWrapper
 
 [![Build Status](https://travis-ci.org/zalando/java-sproc-wrapper.svg)](https://travis-ci.org/zalando/java-sproc-wrapper)
 
+[![Coverage Status](https://coveralls.io/repos/zalando/java-sproc-wrapper/badge.png)](https://coveralls.io/r/zalando/java-sproc-wrapper)
+
 Library to make PostgreSQL stored procedures(SProcs) available through simple Java "SProcService" interfaces including automatic object serialization and deserialization (using typemapper and convention-over-configuration).
 
 Supports horizontal database sharding (partition/access logic lies within application), easy use of pg_advisory_lock through annotations to ensure single Java node execution, configurable statement timeouts per stored procedure, and PostgreSQL types including enums and hstore.