docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # serverless-plugin-typescript
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/graphcool/serverless-plugin-typescript.svg)](https://greenkeeper.io/)
+
 [![serverless](http://public.serverless.com/badges/v3.svg)](http://www.serverless.com) [![npm version](https://badge.fury.io/js/serverless-plugin-typescript.svg)](https://badge.fury.io/js/serverless-plugin-typescript) [![CircleCI](https://circleci.com/gh/graphcool/serverless-plugin-typescript.svg?style=svg)](https://circleci.com/gh/graphcool/serverless-plugin-typescript)
 
 Serverless plugin for zero-config Typescript support