docs: How to setup with Vue 3 Composition API (#1187)

new Vue(...) is changed to createApp(...)

packages/docs/src/guide-composable/setup.md

@@ -16,6 +16,7 @@ yarn add @vue/apollo-composable
 
 ## 2. Connect Apollo Client to Vue
 
+### Vue 2 + composition API instructions
 In your root instance, you need to provide a default Apollo Client instance:
 
 ```js
@@ -31,6 +32,20 @@ const app = new Vue({
 })
 ```
 
+### Vue 3 instructions
+```js
+import { createApp, provide, h } from "vue";
+import { DefaultApolloClient } from '@vue/apollo-composable'
+
+const app = createApp({
+  setup () {
+    provide(DefaultApolloClient, apolloClient)
+  },
+
+  render: h => h(App),
+})
+```
+
 ### Multiple clients
 
 You can also provide multiple Apollo Client instances to be used in your application. In this case, it's recommended to provide a `default` one: