docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # preact-cli-plugin-async [![npm](https://img.shields.io/npm/v/preact-cli-plugin-async.svg)](https://npm.im/preact-cli-plugin-async)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/developit/preact-cli-plugin-async.svg)](https://greenkeeper.io/)
+
 [Preact CLI] plugin that adds optimized support for async/await via [Kneden].
 
 ## Installation