Here we have some rules that will be available with the model when you buy them. So here is your sneak peak for both the Primaris Captain and Primaris Librarian.
I also want to mention that 4chan has been talking about a new Space Marine Codex, coming. Although I have been waiting for more solid information, I have been hearing that the end of August is when we will see it. Information is sketchy at the moment, but I am waiting patiently to discover more. So not really worth up to rumor status yet.
Here are the new Primaris Character Profiles via MiniWars