Document the non zero exit code case.

tests/urchin exit code/non zero exit code when a test fails

@@ -1,4 +1,11 @@
+#!/bin/sh
+
+# Borrow this test which has a failure in it (3 in fact), and
+# run it using urchin:
 ../../urchin "../Counts should be kept of successes and failures./.test" > /dev/null
+
+# Now invert the result, we want _this_ test to fail if the exit
+# code was 0.
 case $? in
   0) exit 8;;
   *) exit 0;;