t/README: Document GIT_TEST_INSTALLED and GIT_TEST_EXEC_PATH

These were added without documentation in 2009-03-16 (6720721).

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -75,6 +75,19 @@ appropriately before running "make".
 	As the names depend on the tests' file names, it is safe to
 	run the tests with this option in parallel.
 
+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
+test-* support programs, templates, and perl libraries are used.
+If your installed git is incomplete, it will silently test parts of
+your built version instead.
+
+When using GIT_TEST_INSTALLED, you can also set GIT_TEST_EXEC_PATH to
+override the location of the dashed-form subcommands (what
+GIT_EXEC_PATH would be used for during normal operation).
+GIT_TEST_EXEC_PATH defaults to `$GIT_TEST_INSTALLED/git --exec-path`.
+
+
 Skipping Tests
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