Add npx note to quick overview

Author: gaearon

README.md

@@ -16,6 +16,8 @@ cd my-app
 npm start
 ```
 
+*([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.<br>
 When you’re ready to deploy to production, create a minified bundle with `npm run build`.