MAINTAINERS: Add btrfs mailing list and myself as reviewer

Since the current code base is mostly from btrfs-progs, anyone
contributing to U-Boot btrfs code could also help us to improve
btrfs-progs and btrfs kernel module.

Also add myself as designated reviewer.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NMarek Behún <marek.behun@nic.cz>

MAINTAINERS

@@ -599,6 +599,8 @@ F:	tools/binman/
 
 BTRFS
 M:	Marek Behun <marek.behun@nic.cz>
+R:	Qu Wenruo <wqu@suse.com>
+L:	linux-btrfs@vger.kernel.org
 S:	Maintained
 F:	cmd/btrfs.c
 F:	fs/btrfs/