Apparently the next codex supplement is for Tau, according to the latest bits of information floating around. It was found in the White Dwarf, and is apparently coming out later this month for the digital version. I am going to imagine that the hard cover copy will be a few weeks behind it.

Please remember that these are not yet verified, but look real.

via Morathi's Darkest Sin
Farsight - A codex: Tau Empire Supplement. Looking at the blurb underneath it seems to indicate it is a Tau Iyanden style extra, and once again mentions rules for using with your Tau.

via Jossy from the Faeit 212 comment sections
I can confirm this is true. Digital release is month of July for Tau supplement: Farsight Enclave. Reading it now in WD.