MAINTAINERS: add URL for virtio-mem

Let's add the status/info page, which is still under construction, however,
already contains valuable documentation/information.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200617104756.6312-1-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: davidhildenbrand

MAINTAINERS

@@ -18503,6 +18503,7 @@ VIRTIO MEM DRIVER
 M:	David Hildenbrand <david@redhat.com>
 L:	virtualization@lists.linux-foundation.org
 S:	Maintained
+W:	https://virtio-mem.gitlab.io/
 F:	drivers/virtio/virtio_mem.c
 F:	include/uapi/linux/virtio_mem.h