How can I show my shop categories on mobile (currently, you have to know to click the "+" symbol)?

Hello!

I just set up my shop (in 7.0 Hayden) with categories, and I'm hoping to find a way to show the product categories on mobile devices. Currently, you have to know to click the + symbol, and I would rather not assume people know to do that. 

On the desktop, it shows on a sidebar on the left and seems to work well.

Thanks in advance!