docs: wip external like pinia

Author: posva

docs/guide/subscriptions-external.md

@@ -0,0 +1,28 @@
+# Binding Firebase Reference to existing Vue Refs
+
+When you need to bind a Firebase reference to existing Vue `ref()`, you can pass it as the `target` option:
+
+```ts
+todos // Ref<Todo[]>
+useCollection(todoListRef, { target: todos })
+```
+
+## Pinia
+
+If you are using [Pinia](https://pinia.vuejs.org), you can directly use the `useCollection` function within [setup stores](https://pinia.vuejs.org/cookbook/composables.html#setup-stores):
+
+```ts
+import { defineStore } from 'pinia'
+
+export const useTodoStore = defineStore('todos', () => {
+  const todos = useCollection(todoListRef)
+  
+  return { todos }
+})
+```
+
+Note you will still have to follow the [Firebase API](https://firebase.google.com/docs/firestore/manage-data/structure-data) (e.g. `addDoc()`, `updateDoc()`, etc) to update the data.
+
+## TypeScript
+
+TODO: use type helper to add properties of converter