docs: add missing Vue.use(VueI18n) (#19)

In the newly released v1 and v2, `Vue.use(VueI18n)` is removed from `createI18n`, so users should call it manually. Now `createI18n` does not need and accept a second parameter which is also a breaking change in unit test.

Author: kingyue737

README.md

@@ -30,9 +30,12 @@ npm i @vue/composition-api
 ```js
 // main.js
 import Vue from 'vue'
+import VueI18n from 'vue-i18n'
 import { createI18n } from 'vue-i18n-composable'
 import App from './App.vue'
 
+Vue.use(VueI18n)
+
 const i18n = createI18n({
   locale: 'ja',
   messages: {
@@ -79,11 +82,13 @@ export default defineComponent({
 ```js
 // main.js
 import Vue from 'vue'
+import VueI18n from 'vue-i18n'
 import VueCompositionAPI, { createApp } from '@vue/composition-api'
 import { createI18n } from 'vue-i18n-composable'
 import App from './App.vue'
 
 Vue.use(VueCompositionAPI)
+Vue.use(VueI18n)
 
 const i18n = createI18n({
   locale: 'ja',