MAINTAINERS: Add reviewer entry type and make me a s390 reviewer

As I missed some mails that introduced problems on s390 and their
fixes, this might help to broaden review coverage.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20180927112835.134812-1-frankja@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index d07a4cf..d195826 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13,6 +13,8 @@
 Descriptions of section entries:
 
 	M: Mail patches to: FullName <address@domain>
+	R: Designated reviewer: FullName <address@domain>
+	   These reviewers should be CCed on patches.
 	L: Mailing list that is relevant to this area
 	W: Web-page with status/info
 	Q: Patchwork web based patch tracking system site
@@ -79,6 +81,7 @@
 S390X
 M: Thomas Huth <thuth@redhat.com>
 M: David Hildenbrand <david@redhat.com>
+R: Janosch Frank <frankja@linux.ibm.com>
 L: kvm@vger.kernel.org
 F: s390x/*
 F: lib/s390x/*