add japanese translation info (vuejs#706)

Author: kazupon

README.md

@@ -22,7 +22,10 @@ Translation for this documentation project are currently mantained in separate r
 
 ### Japanese
 
+Japanese translation is maintained by [Vue.js japan user group](https://github.com/vuejs-jp)
+
 * Translation Repo - [/vuejs/jp.vuejs.org](https://github.com/vuejs/jp.vuejs.org)
+* Primary maintainer - [kazupon](https://github.com/kazupon)
 
 ### Russian