doc: add gitter chat link to readme

Closes #470

README.md

@@ -3,6 +3,7 @@

[![Build Status](https://travis-ci.org/pgjdbc/pgjdbc.png)](https://travis-ci.org/pgjdbc/pgjdbc)
[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.postgresql/postgresql/badge.svg)](https://maven-badges.herokuapp.com/maven-central/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)

This is a simple readme describing how to compile and use the PostgreSQL JDBC driver.