Steam Workshop support is released in update 1.4. Custom courses can now be uploaded and shared, so get creating!

Changelog:

• Added Steam Workshop support
  
• Pre-existing terrain maps can now be used for editor courses
  
• Added (lenient) build limits to courses to prevent potential issues with custom content
  
• Improved AI pathfinding to handle less-optimal waypoint layouts
  
• Bugfixes
  

As always, it's hard to thoroughly QA test big updates like these despite my best efforts, so please report any bugs you encounter. Happy racing!