It's not the case that any USB WiFi dongle will work -- you need to find one with OS X drivers.
As an example, often the ones based on a Realtek chip will have the drivers. Though, you may need to
poke around a little to find Realtek USB WiFi drivers that work under El Capitan -- many vendors have not posted the update from Realtek that supports 10.11. The drivers are basically generic to the chip, so you can use a Realtek driver release that supports the chip even if the vendor is not offering it from the Web site yet.
Still, best case is that you find a model that offers OS X 10.11 support from the start.