Document HOME environment variable

Document HOME environment variable

Git documentation refers to $HOME and $XDG_CONFIG_HOME often, but does
not specify how or where these values come from on Windows where neither
is set by default. The new documentation reflects the behavior of
setup_windows_environment() in compat/mingw.c.

Signed-off-by: Alejandro Barreto <alejandro.barreto@ni.com>
Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>