Minor update to Subpro.txt

Minor update to Subpro.txt

This is ancient in git timescale, but we see requests for
subprojects support every now and then.  The document talks
about a possible UI level semantics and core level support.

Most of the core level support is in the "bind commit" updates
topic branch, except a few.  Most notably, "update-index --bind"
and "update-index --unbind" still needs to be written.

Signed-off-by: Junio C Hamano <junkio@cox.net>