Add Gitter badge (#17)

Author: gitter-badger

README.md

@@ -1,5 +1,7 @@
 serverless-dynamodb-local
 =================================
+
+[![Join the chat at https://gitter.im/99xt/serverless-dynamodb-local](https://badges.gitter.im/99xt/serverless-dynamodb-local.svg)](https://gitter.im/99xt/serverless-dynamodb-local?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![serverless](http://public.serverless.com/badges/v3.svg)](http://www.serverless.com)
 [![npm version](https://badge.fury.io/js/serverless-dynamodb-local.svg)](https://badge.fury.io/js/serverless-dynamodb-local)
 [![license](https://img.shields.io/npm/l/serverless-dynamodb-local.svg)](https://www.npmjs.com/package/serverless-dynamodb-local)