docs(readme): add Greenkeeper badge

greenkeeper[bot] · travi · commit 7cb2f9f9e558 · 2018-08-21T00:06:22.000-05:00
diff --git a/README.md b/README.md
@@ -1,5 +1,7 @@
 # commitlint-config-traverson
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/traverson/commitlint-config-traverson.svg)](https://greenkeeper.io/)
+
 shareable config for [commitlint](https://github.com/marionebl/commitlint)
 
 <!-- status badges -->
