The main documentation for the wifi_ddwrt package is the wifi_ddwrt page on the ROS wiki.
For Hydro and newer versions of ROS, use the hydro-devel branch:
git clone -b hydro-devel https://github.com/ros-drivers/wifi_ddwrt.git
For Groovy and older version of ROS, use the master branch:
git clone -b master https://github.com/ros-drivers/wifi_ddwrt.git