README: update stale and/or incorrect information

Ramkumar Ramachandra noticed that the old address for the marc
archive no longer works.  Update it to its marc.info address,
and also refer to the gmane site.

Remove the reference to "note from the maintainer", which is not
usually followed by any useful discussion on status, direction nor
tasks.

Also replace the reference to "What's in git.git" with "What's
cooking".

Signed-off-by: Junio C Hamano <gitster@pobox.com>