t/README: Document the predefined test prerequisites

The README for the test library suggested that you grep the
test-lib.sh for test_set_prereq to see what the preset prerequisites
were.

Remove that bit, and write a section explaining all the preset
prerequisites. Most of the text was lifted from from Junio C Hamano
and Johannes Sixt, See the "Tests in Cygwin" thread in May 2009 for
the originals:

http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118385
http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118434

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>