Add important note to 1989/fubar/README.md

In the judges' remarks they refer to ouroboros.c as being an executable
but it's not in the repo. This is because it's created by the program
itself. It happens to be in .gitignore but this note should make it less
confusing and prevent people from thinking that a file is actually
missing.

1989/fubar/README.md

@@ -33,10 +33,13 @@ Run this with a single digit argument (or wait a long time).
 The blank line at the beginning of the source is mandatory.
 Do you know why?
 
-NOTE: don't try running [ouroboros.c](ouroboros.c) (note that it's executable)
+NOTE: don't try running `ouroboros.c` (note that it's executable)
 directly with an arg as it'll turn into an infinite loop. The same will happen
 with larger numbers.
 
+NOTE: the file `ouroboros.c` is created by the program itself so this is why it
+is not in the repository.
+
 ## Author's remarks:
 
 In a nutshell, this is probably the slowest and most