docs: submitting-patches: Advertise b4

b4 is now widely used and is quite helpful for a lot of the things that
submitting-patches covers, let's advertise it to submitters to try to make
their lives easier and reduce the number of procedural issues maintainers
see.

Reviewed-by: Shuah Khan <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Acked-by: Konstantin Ryabitsev <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/process/submitting-patches.rst

···
 and not in some internal, accessible only to you tree - otherwise it
 would be worthless.
 
+Tooling
+-------
+
+Many of the technical aspects of this process can be automated using
+b4, documented at <https://b4.docs.kernel.org/en/latest/>. This can
+help with things like tracking dependencies, running checkpatch and
+with formatting and sending mails.
+
 References
 ----------