Described in commit 97e203fd89e7cb54c7ce96ce779b2e8cb6ac1374 Linux was not including any license files by mistake.
OpenVR SDK
OpenVR is an API and runtime that allows access to VR hardware from multiple vendors without requiring that applications have specific knowledge of the hardware they are targeting. This repository is an SDK that contains the API and samples. The runtime is under SteamVR in Tools on Steam.
Documentation
Documentation for the API is available on the Github Wiki
More information on OpenVR and SteamVR can be found on http://steamvr.com