add note for vue-cli 2

Author: Kocal

README.md

@@ -23,7 +23,7 @@ This template allows you to quickly start a web extension containing:
 
 - Node.js >= 8 and npm >= 5
 - [git](https://git-scm.com)
-- [vue-cli](https://github.com/vuejs/vue-cli)
+- [vue-cli 2](https://github.com/vuejs/vue-cli/tree/v2)
 
 ## Usage