t/README: clarify test_must_fail description

Some have found the wording of the description to be somewhat ambiguous
with respect to when it is desirable to use test_must_fail instead of
"! <git-command>".  Tweak the wording somewhat to hopefully clarify that
it is _because_ test_must_fail can detect segmentation fault that it is
desirable to use it instead of "! <git-command>".

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: drafnel

t/README

@@ -275,6 +275,14 @@ library for your script to use.
    Merges the given rev using the given message.  Like test_commit,
    creates a tag and calls test_tick before committing.
 
+ - test_must_fail <git-command>
+
+   Run a git command and ensure it fails in a controlled way.  Use
+   this instead of "! <git-command>".  When git-command dies due to a
+   segfault, test_must_fail diagnoses it as an error; "! <git-command>"
+   treats it as just another expected failure, which would let such a
+   bug go unnoticed.
+
 Tips for Writing Tests
 ----------------------