t/README: document --root option We've had this option since f423ef5 (tests: allow user to specify trash directory location, 2009-08-09). Make it easier to look up :-) Signed-off-by: Thomas Rast <trast@student.ethz.ch> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>