t/README: document --root option
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 <18f23ef7096473d837b7404952f13710423b8b63@student.ethz.ch>
Acked-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git