add vue3 quickstart instruction

Author: Tobertet

docs/vue-testing-library/intro.mdx

@@ -20,9 +20,14 @@ In short, Vue Testing Library does three things:
 
 ## Quickstart
 
+If using Vue2
 ```
 npm install --save-dev @testing-library/vue
 ```
+If using Vue3
+```
+npm install --save-dev @testing-library/vue@next
+```
 
 You can now use all of `DOM Testing Library`'s `getBy`, `getAllBy`, `queryBy`
 and `queryAllBy` commands. See here the