Though it hasn’t been officially mentioned by Square Enix or Crystal Dynamics, info shows Rise of the Tomb Raider is coming to Steam in January 2016. Though the game has been launched already this past November on Xbox One and Xbox 360, fans of the PC version only knew about an ‘Early 2016’ launch.
A post on Steam’s news page currently says “Rise of the Tomb Raider is coming to Steam this January!” and the game’s page also mentions its January availability. It will still be a long wait for gamers who would like to play the game on the PS4, once the one-year console exclusivity ends, but PC owners are not that far away from playing it.
[Source]: Steam: Coming Soon to Steam – Rise of the Tomb Raider; Rise of the Tomb Raider.