docs(readme): add Greenkeeper badge (#1)

greenkeeper[bot] · JounQin · commit bd14665636ca · 2017-12-30T03:34:29.000+08:00
diff --git a/README.md b/README.md
@@ -1,2 +1,4 @@
 # react-server-render
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/JounQin/react-server-render.svg)](https://greenkeeper.io/)
 simple React SSR solution inspired by vue-server-render
