add some readme

README.md

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+## Backup script for libvirt/virsh vms on zfs
+
+### Usage
+```
+libvirt-zsh-backup.sh -v VmName -s SourceDataset -t TargetHost -d TargetDataset -l LIMIT
+```
+
+### Parameters
+#### Script
+```
+-v: Name of VM
+-t: Hostname of backup target
+-s: Source zfs fileset
+-d: Destination zfs fileset
+-l: Rate limit for zfs send/recv in MB/s
+```
+
+#### ZFS
+```
+backup:days	Count of days to keep Backups
+```