Add Dev badge (#3)

Dev badge is released. Thank you for using Dev :)

Author: aggre

README.md

@@ -2,6 +2,7 @@
 
 [![CircleCI](https://circleci.com/gh/kazupon/vue-i18n-extensions/tree/dev.svg?style=svg)](https://circleci.com/gh/kazupon/vue-i18n-extensions/tree/dev)
 [![npm](https://img.shields.io/npm/v/vue-i18n-extensions.svg)](https://www.npmjs.com/package/vue-i18n-extensions)
+[![vue-i18n-extensions Dev Token](https://badge.devtoken.rocks/vue-i18n-extensions)](https://devtoken.rocks/package/vue-i18n-extensions)
 
 > Extensions for vue-i18n