t/README: justify why "! grep foo" is sufficient

We require use of test_must_fail to check expected non-zero exit by
Git itself, but discourage test_must_fail to be used for checking
exit status of non Git commands that are supplied by the system.
The current text explains the reason for the former but not the
latter.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/README

@@ -335,7 +335,8 @@ Don't:
    dies in an unexpected way (e.g. segfault).
 
    On the other hand, don't use test_must_fail for running regular
-   platform commands; just use '! cmd'.
+   platform commands; just use '! cmd'.  We are not in the business
+   of verifying that the world given to us sanely works.
 
  - use perl without spelling it as "$PERL_PATH". This is to help our
    friends on Windows where the platform Perl often adds CR before