added comment on nbd release

README.md

@@ -32,7 +32,7 @@ img="/var/lib/libvirt/images/ts-linux-testing.qcow2"
 qemu-img create -f qcow2 -o size=20G $img  # create image
 chown libvirt-qemu:libvirt-qemu $img       # make accessable for qemu
 qemu-nbd -c /dev/nbd0 $img  # create device from image
-qemu-nbd -d /dev/nbd0       # release device
+qemu-nbd -d /dev/nbd0       # release device, after the installation is completed
 ```
 
 #### Partition schema