The MD update itself isn't too exciting with just having some clustered RAID1 and RAID5 journal updates. There's also a Year 2038 fix and other minor changes. The details on that can be found via the pull request.
This is Neil Brown's last MD pull request as he announced a few weeks ago he's resigning. He wrote much of the original MD/mdadm code while at SUSE and has been the kernel maintainer of the Linux Software RAID code since 2001. However, he's been losing interest in recent years and feels that it's time to resign, per his announcement.
Taking over for Neil will likely be a few existing MD developers from various companies to form a maintainership team around the subsystem.