Update 2019/03/06: UNET is now deprecated so this solution might not be compatible with newer versions of Unity. See the UNET Deprecation FAQ for more information.
Example integration of Unity's UNET HLAPI and Steamworks P2P networking using Steamworks.NET.
Last tested with Unity 2017.2.0p2 on 26 November 2017
Read more here: https://blog.spacewavesoftware.com/gamedev/2017-10-28-unity-unet-hlapi-and-steam-p2p-networking/