To have a list of Gems that are not compatible and those that are maybe with a link to the current rails edge branch e.g. 4.2
To add at the bottom of the Rails Edge guide i.e. http://edgeguides.rubyonrails.org/4_2_release_notes.html
This would be very helpful to know when Devise or Bullet works with Rails 4.2, it would help people make a decision when is the time to upgrade. It might also add momentum to Gem Developers to update the Gem, or encourage others to get involved and upgrading it.