diff --git a/t/README b/t/README
index dcd3ebb5f2..0e4e8d8862 100644
--- a/t/README
+++ b/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