The Tau Empire is coming to Battlefleet Gothic: Armada—tomorrow, in fact, although only in beta. The Tau fleet will be free for everyone who either preordered the game or purchased it during the first two months of release (prior to June 21, in other words) but the beta will be open to everyone who wants to give it a go, regardless of when or how they bought it.
The Tau possess highly advanced technology that gives them the ability to inflict devastating damage to enemy fleets long before they reach boarding distances, and their Torpedoes and Ordnances can be improved even further through a wide array of upgrades. The Tau also have a “total dedication to the Greater Good philosophy,” although I'm unclear as to what sort of practical value its societal doctrines will bring to a life-and-death throwdown with the Orks.
The instructions for accessing the Tau beta are available on Steam, and are very straightforward: Right-click Battlefleet Gothic: Armada in your Steam library, select Properties, hit the Betas tab, and then select the Tau Public Beta option from the dropdown menu. After the update, you're ready to go.
The Battlefleet Gothic: Armada Tau beta begins on September 8, and is expected to run until early next week. A full launch date hasn't been announced, nor has pricing, but developer Tindalos Interactive said it will be out before the end of September and I'd expect it to sell for the same price as the Space Marines DLC, which goes for $7/£5.