README: update ray4-c description

README.md

@@ -20,11 +20,11 @@ Wire4
 reads in arbitrary 4D wireframes from a simple text file and displays them interactively. The code
 has not been maintained, though, and will not run on most modern computers.
 
-Ray4
------
-[`ray4-c`][] is the original 4D raytracer. It is written in standard C, and reads from and writes to
-standard output, so should be able to run on most computers. The output is a 3D RGB image that can
-be displayed with a variety of methods.
+Ray4-c
+-------
+[`ray4-c`][] is the original 4D raytracer, written in standard C. It can read from standard input
+and writes to standard output, so should be able to run on most computers. The output is a 3D RGB
+image that can be displayed with a variety of methods. See [`ray4-c/craig/`][craig] for some tools.
 
 
 GitHub Notes
@@ -46,5 +46,6 @@ visualization work of their own. Here are links to other work I've seen.
 [_Four-Space Visualization of 4D Objects_]: https://hollasch.github.io/ray4/Four-Space_Visualization_of_4D_Objects.html
 [the discussions section]:                  https://github.com/hollasch/ray4/discussions/
 [`ray4-c`]:                                 ray4-c/README.md
+[craig]:                                    ray4-c/craig/README.md
 [`wire4`]:                                  wire4/README.md
 [`thesis/`]:                                thesis/README.md