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>

t/README

@@ -84,6 +84,12 @@ appropriately before running "make".
 	implied by other options like --valgrind and
 	GIT_TEST_INSTALLED.
 
+--root=<directory>::
+	Create "trash" directories used to store all temporary data during
+	testing under <directory>, instead of the t/ directory.
+	Using this option with a RAM-based filesystem (such as tmpfs)
+	can massively speed up the test suite.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various