Add reference to 2019/karns/demo.sh in thanks file

thanks-for-fixes.md

@@ -1641,6 +1641,9 @@ appears to fix the problem in both macOS and linux. Perhaps this is the problem
 that the author reported where it sometimes segfaults but Cody did not try
 debugging it since it works with `-O0`.
 
+He also added the script [demo.sh](2019/karns/demo.sh) to showcase the entry a
+bit more easily.
+
 ## [2020/kurdyukov2](2020/kurdyukov2/prog.c) ([README.md](2020/kurdyukov2/README.md))
 
 Cody added `-L`/`-I` paths to the Makefile to let this compile more easily if