docs(readme): add Greenkeeper badge

greenkeeper[bot] · BerkeleyTrue · commit c9aa6ece7c6d · 2018-05-31T10:53:56.000-07:00
diff --git a/README.md b/README.md
@@ -1,4 +1,6 @@
 # Warning [![npm version](https://badge.fury.io/js/warning.svg)](https://badge.fury.io/js/warning)
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/BerkeleyTrue/warning.svg)](https://greenkeeper.io/)
 A mirror of Facebook's [Warning](https://github.com/facebook/fbjs/blob/master/packages/fbjs/src/__forks__/warning.js)
 
 
