chore: add coverage badge (#646)

add coverage badge

README.md

@@ -2,6 +2,7 @@
# PostgreSQL JDBC driver

[![Build Status](https://travis-ci.org/pgjdbc/pgjdbc.svg?branch=master)](https://travis-ci.org/pgjdbc/pgjdbc)
[![codecov.io](http://codecov.io/github/pgjdbc/pgjdbc/coverage.svg?branch=master)](http://codecov.io/github/pgjdbc/pgjdbc?branch=master)
[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.postgresql/postgresql/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.postgresql/postgresql)
[![Javadocs](http://javadoc.io/badge/org.postgresql/postgresql.svg)](http://javadoc.io/doc/org.postgresql/postgresql)
[![Join the chat at https://gitter.im/pgjdbc/pgjdbc](https://badges.gitter.im/pgjdbc/pgjdbc.svg)](https://gitter.im/pgjdbc/pgjdbc?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)