docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -3,6 +3,8 @@
 # GrAMPS GraphQL Data Source Base
 [![Build Status](https://travis-ci.org/gramps-graphql/data-source-base.svg?branch=master)](https://travis-ci.org/gramps-graphql/data-source-base) [![Maintainability](https://api.codeclimate.com/v1/badges/ac264833fac1fbd1afe0/maintainability)](https://codeclimate.com/github/gramps-graphql/data-source-base/maintainability) [![Test Coverage](https://api.codeclimate.com/v1/badges/ac264833fac1fbd1afe0/test_coverage)](https://codeclimate.com/github/gramps-graphql/data-source-base/test_coverage) [![npm version](https://img.shields.io/npm/v/@gramps/data-source-base.svg?style=flat)](https://www.npmjs.com/package/@gramps/data-source-base) [![Greenkeeper badge](https://badges.greenkeeper.io/gramps-graphql/data-source-base.svg)](https://greenkeeper.io/)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/gramps-graphql/data-source-base.svg)](https://greenkeeper.io/)
+
 This is a minimal example and boilerplate for a GrAMPS data source. Inside, you’ll find:
 
  -  **Connector** — how to access the data source (e.g. a REST API)