Docs/process/email-clients: Document HacKerMaiL

Docs/process/email-clients: Document HacKerMaiL

HacKerMaiL (hkml) [1] is a simple tool for mailing lists-based
development workflows such as that for most Linux kernel subsystems.  It
is actively being maintained by DAMON maintainer, and recommended for
DAMON community[2].  Add a simple introduction of the tool on the
email-clients document, too.

[1] https://github.com/sjp38/hackermail
[2] https://lore.kernel.org/20240621170353.BFB83C2BBFC@smtp.kernel.org

Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240624185312.94537-8-sj@kernel.org