Vermintide 2 Steam Workshop has officially opened! It can be accessed from the Community Hub in your Steam client.
From here you are able to download mods to be played in the Modded Realm of the game.
Switching between Official Realm and Modded Realm can be done through the launcher. Modded Realm has a no holds barred stance towards mods and custom content, but on the other hand will not be able to give new drops, allow challenge/quest completion or grant XP.
For more info about the Modded Realm, creating mods yourself and future mods sanctioned for the Official Realm: http://www.vermintide.com/mods_guide/
official blog post http://www.vermintide.com/news/mod-support-dev-blog/