documentation: README.md: add common rsync options

README.md

@@ -300,7 +300,12 @@ First create a btrfs subvolume on the backup server:
 In your daily cron script, prior to running btrbk, sync your source to
 `myhost_sync`, something like:
 
-    rsync -a --inplace --delete -e ssh myhost.mydomain.com:/data/ /mnt/btr_backup/myhost_sync/
+    #!/bin/sh
+    rsync -az --delete \
+          --inplace --numeric-ids --acls --xattrs \
+          -e 'ssh -i /etc/btrbk/ssh/id_rsa' \
+          myhost.mydomain.com:/data/ \
+          /mnt/btr_backup/myhost_sync/
 
 Then run btrbk, with myhost_sync configured *without any targets*: