Documentation: mention the amlog in howto/maintain-git.txt

Documentation: mention the amlog in howto/maintain-git.txt

Part of the maintainer's job is to keep up-to-date and publish the
'amlog' which stores a mapping between a patch's 'Message-Id' e-mail
header and the commit generated by applying said patch.

But our Documentation/howto/maintain-git.txt does not mention the amlog,
or the scripts which exist to help the maintainer keep the amlog
up-to-date.

(This bit me during the first integration round I did as interim
maintainer[1] involved a lot of manual clean-up. More recently it has
come up as part of a research effort to better understand a patch's
lifecycle on the list[2].)

Address this gap by briefly documenting the existence and purpose of the
'post-applypatch' hook in maintaining the amlog entries.

[1]: https://lore.kernel.org/git/Y19dnb2M+yObnftj@nand.local/
[2]: https://lore.kernel.org/git/CAJoAoZ=4ARuH3aHGe5yC_Xcnou_c396q_ZienYPY7YnEzZcyEg@mail.gmail.com/

Suggested-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>