The Sovrin DID Method Specification is published by the Sovrin Foundation in the Sovrin github repository.
As of the start of the Fall 2018 Rebooting the Web of Trust conference (6 March 2018), the current version of the Sovrin DID Method Specification is currently in a fork maintained by Mike Lodder as the lead author. We expect this fork to be merged back into the Sovrin master shortly.