concepts.md - include Vue.use(Vuex) call

Author: jbruni

docs/en/concepts.md

@@ -21,8 +21,11 @@ Why do we differentiate between *mutations* and *actions*, rather then just simp
 Creating a Vuex store is pretty straightforward - just put the aforementioned ingredients together:
 
 ``` js
+import Vue from 'vue'
 import Vuex from 'vuex'
 
+Vue.use(Vuex)
+
 const store = new Vuex.Store({
   state: { ... },
   mutations: { ... },