Add docs about the use-npm flag in create next app

docs/api-reference/create-next-app.md

@@ -18,6 +18,7 @@ yarn create next-app
 
 - **-e, --example [name]|[github-url]** - An example to bootstrap the app with. You can use an example name from the [Next.js repo](https://github.com/vercel/next.js/tree/master/examples) or a GitHub URL. The URL can use any branch and/or subdirectory.
 - **--example-path [path-to-example]** - In a rare case, your GitHub URL might contain a branch name with a slash (e.g. bug/fix-1) and the path to the example (e.g. foo/bar). In this case, you must specify the path to the example separately: `--example-path foo/bar`
+- **--use-npm** - Explicitly tell the CLI to bootstrap the app using NPM. By default, the CLI will detect if Yarn is installed and use it if it's available.
 
 ### Why use Create Next App?