Add use-npm CLI flag docs (#17803)

Related issue: https://github.com/vercel/next.js/issues/10647#issuecomment-623703149

Sometimes we have both Yarn and NPM installed and want to explicitly bootstrap an app with the CLI using NPM. Having this flag documented could help people understand the package manager behavior and select NPM if that's their preference.

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. Yarn will be used by default if it's installed
 
 ### Why use Create Next App?