Add vuex merging strategy as an example in mixins

src/guide/mixins.md

@@ -125,3 +125,18 @@ For most object-based options, you can simply use the same strategy used by `met
 var strategies = Vue.config.optionMergeStrategies
 strategies.myOption = strategies.methods
 ```
+
+A more advanced example can be found on [Vuex](https://github.com/vuejs/vuex)'s merging strategy:
+
+``` js
+const merge = Vue.config.optionMergeStrategies.computed
+Vue.config.optionMergeStrategies.vuex = (toVal, fromVal) => {
+  if (!toVal) return fromVal
+  if (!fromVal) return toVal
+  return {
+    getters: merge(toVal.getters, fromVal.getters),
+    state: merge(toVal.state, fromVal.state),
+    actions: merge(toVal.actions, fromVal.actions)
+  }
+}
+```