Description
This uses a JS library and works without a second copy of the menu in the HTML, and you don't need to make any changes to your existing navigation menu.
Look at the README file for instructions.
You need to install mm_menu and superfish in your libraries folder.
Note you're forced with the mobile version of the menu to have it expand to full depth (although the hack/workaround, if you want a top-level menu only, is to hide the lower elements in CSS)
For hand-crated CSS / minimal javascript solutions, see https://css-tricks.com/responsive-menu-concepts/