t/README: Add a note about the dangers of coverage chasing

t/README: Add a note about the dangers of coverage chasing

Having no coverage at all is almost always a bad sign, but trying to
attain 100% coverage everywhere is usually a waste of time. Add a
paragraph to explain this to future test writers.

Inspired-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>