t/README: document GIT_TEST_DEFAULT_HASH

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -421,6 +421,10 @@ GIT_TEST_DISALLOW_ABBREVIATED_OPTIONS=<boolean>, when true (which is
 the default when running tests), errors out when an abbreviated option
 is used.
 
+GIT_TEST_DEFAULT_HASH=<hash-algo> specifies which hash algorithm to
+use in the test scripts. Recognized values for <hash-algo> are "sha1"
+and "sha256".
+
 Naming Tests
 ------------