docs(readme): add Greenkeeper badge (#2)

fixes #1

Author: frux

README.md

@@ -1,4 +1,6 @@
 # unity-table
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/auru/unity-table.svg)](https://greenkeeper.io/)
 > Table component for React
 
 * [SimpleTable](#simpletable)