Hello Tim!
Happy 2018! 
The best source of documentation is current found in comments of the UseITK.cmake file. 
h/t @jcfr
In summary, these files, when included in an executable or library, will register the available itk::ImageIOBase classes to the object factory, so itk::ImageFileReader and itk::ImageFileWriter can find and use them.
This, of course, depends on the available ImageIO modules that ITK has available (depends on ITK’s build options and any external modules). The infrastructure was improved even further in ITK 4.13.0, which makes the use of CMake even more appealing. h/t @fbudin
HTH,
Matt