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

Author: sjp38

Documentation/process/email-clients.rst

@@ -350,3 +350,12 @@ although tab2space problem can be solved with external editor.
 
 Another problem is that Gmail will base64-encode any message that has a
 non-ASCII character. That includes things like European names.
+
+HacKerMaiL (TUI)
+****************
+
+HacKerMaiL (hkml) is a public-inbox based simple mails management tool that
+doesn't require subscription of mailing lists.  It is developed and maintained
+by the DAMON maintainer and aims to support simple development workflows for
+DAMON and general kernel subsystems.  Refer to the README
+(https://github.com/sjp38/hackermail/blob/master/README.md) for details.