Add note for global install of CLI (facebook#6157)

* Add note for global install of CLI

Closes facebook#6140.

docusaurus/docs/getting-started.md

@@ -14,6 +14,8 @@ cd my-app
 npm start
 ```
 
+> If you've previously installed `create-react-app` globally via `npm install -g create-react-app`, we recommend you uninstall the package using `npm uninstall -g create-react-app` to ensure that `npx` always uses the latest version.
+
 _([npx](https://medium.com/@maybekatz/introducing-npx-an-npm-package-runner-55f7d4bd282b) comes with npm 5.2+ and higher, see [instructions for older npm versions](https://gist.github.com/gaearon/4064d3c23a77c74a3614c498a8bb1c5f))_
 
 Then open [http://localhost:3000/](http://localhost:3000/) to see your app.