The next version of RimWorld, Alpha 14, will be released on Steam Early Access on July 15, 2016.
You can add it to your Steam wish list on the RimWorld Steam page.
On Steam, the game will have full Workshop support, for sharing mods and scenarios. It will also support Steam Cloud.
Non-Steam users will, of course, still be able to access your DRM-free downloads as you always have. They’ll update just as before.