Added maven badge to README

Author: sapessi

README.md

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-# Serverless Java container [![Build Status](https://travis-ci.org/awslabs/aws-serverless-java-container.svg?branch=master)](https://travis-ci.org/awslabs/aws-serverless-java-container) [![Help](http://img.shields.io/badge/help-gitter-E91E63.svg?style=flat-square)](https://gitter.im/awslabs/aws-serverless-java-container)
+# Serverless Java container [![Build Status](https://travis-ci.org/awslabs/aws-serverless-java-container.svg?branch=master)](https://travis-ci.org/awslabs/aws-serverless-java-container) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.amazonaws.serverless/aws-serverless-java-container/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.amazonaws.serverless/aws-serverless-java-container) [![Help](http://img.shields.io/badge/help-gitter-E91E63.svg?style=flat-square)](https://gitter.im/awslabs/aws-serverless-java-container)
 The `aws-serverless-java-container` makes it easy to run Java applications written with frameworks such as [Spring](https://spring.io/), [Spring Boot](https://projects.spring.io/spring-boot/), [Jersey](https://jersey.java.net/), or [Spark](http://sparkjava.com/) in [AWS Lambda](https://aws.amazon.com/lambda/).
 
 Serverless Java Container natively supports API Gateway's proxy integration models for requests and responses, you can create and inject custom models for methods that use custom mappings.