docs(create-app): add npm@7 and yarn usage (#2513)

Author: songkeys

packages/create-app/README.md

@@ -22,7 +22,14 @@ Then follow the prompts!
 You can also directly specify the project name and the template you want to use via additional command line options. For example, to scaffold a Vite + Vue project, run:
 
 ```bash
+# npm 6.x
 npm init @vitejs/app my-vue-app --template vue
+
+# npm 7+, extra double-dash is needed:
+npm init @vitejs/app my-vue-app -- --template vue
+
+# yarn
+yarn create @vitejs/app my-vue-app --template vue
 ```
 
 Currently supported template presets include: