This repo contains a driver originally called rtl8821AU_linux_v4.3.14_13455.20150212_BTCOEX20150128-51 with additional patches to get it to build on Ubuntu xenial 16.04.
I originally got the source code off a cd that came with this usb wifi adapter.
I tested on Ubuntu 16.04.2 LTS with kernel 4.4.0-64-generic using a "Heiyo Network Wi-Fi Dongle 802.11ac"
sudo make clean
make -j 8
sudo make install
The driver license appears to be GPLv2
The repository diederikdehaas/rtl8812AU appears to have newer and older versions of the RealTek driver. It also has a version supporting dkms.