Rework the introduction to the Submodules section to explain why
someone would use them, and fix up submodule references from the
tree-object and todo sections.
Signed-off-by: Michael Smith <msmith@cbnco.com> Acked-by: J. Bruce Fields <bfields@citi.umich.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>