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 <skhan@linuxfoundation.org>
Signed-off-by: Mark Brown <broonie@kernel.org>