MAINTAINERS: Begin module maintainer transition

Being a Linux kernel maintainer has been my proudest professional
accomplishment, spanning the last 19 years.  But now we have a surfeit
of excellent hackers, and I can hand this over without regret.

I'll still be around as co-maintainer for another cycle, but Jessica
is now the one to convince if you want your patches applied.  She
rocks, and is far more timely than me too!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Jessica Yu <jeyu@redhat.com>