Update bugs/README for 1998/schnitzi

I removed from bugs.md the suggestion that it does not work for macOS as
I fixed that. I also slightly reworded the entry in bugs.md (about
warnings to ignore).

Add to README.md summary of problem.

1998/schnitzi/README.md

@@ -29,7 +29,8 @@
 	./sort < data
 
 [Cody Boone Ferguson](/winners.html#Cody_Boone_Ferguson) fixed a segfault in
-this entry. Thank you Cody for your assistance!
+this entry. The problem was invalid data types. Thank you Cody for your
+assistance!
 
 ## Judges' comments