Merge pull request #296 from pushkyn/readme-maven-badge

Update readme - added maven central badge

Author: Matt Bernier

README.md

@@ -1,5 +1,6 @@
 ![SendGrid Logo](https://uiux.s3.amazonaws.com/2016-logos/email-logo%402x.png)
 [![Travis Badge](https://travis-ci.org/sendgrid/sendgrid-java.svg?branch=master)](https://travis-ci.org/sendgrid/sendgrid-java)
+[![Maven Central](https://img.shields.io/maven-central/v/com.sendgrid/sendgrid-java.svg)](http://mvnrepository.com/artifact/com.sendgrid/sendgrid-java)
 [![Email Notifications Badge](https://dx.sendgrid.com/badge/java)](https://dx.sendgrid.com/newsletter/java)
 [![Twitter Follow](https://img.shields.io/twitter/follow/sendgrid.svg?style=social&label=Follow)](https://twitter.com/sendgrid)
 [![GitHub contributors](https://img.shields.io/github/contributors/sendgrid/sendgrid-java.svg)](https://github.com/sendgrid/sendgrid-java/graphs/contributors)