README.md: explain restore method for saved files on LTFS tape

Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

README.md

@@ -304,6 +304,34 @@ FUSE). Read and write access can be managed using common OS tools.
 An open-source implementation can be found at
 [LinearTapeFileSystem](https://github.com/LinearTapeFileSystem/ltfs).
 
+## Restore
+
+### From Tape
+
+When `dsnap-sync` did save the data with method `btrfs-archive`, you will find
+the corresponding data in a snapper compatible directory structure on the tape.
+
+The structure may look like:
+
+└── backups
+	└── @<server-name>
+		├── archive-<subvol-name>
+		│   └── <subvol-id>
+		│       ├── <subvol-id>_full.btrfs
+		│       └── info.xml
+
+The file `info.xml` provide the metadata corresponding to the snapshot.
+The data of the snapshot is stored in the file `<subvol-id>_full.btrfs`.
+This file has to be decrypted with btrfs tool `btrfs-send` to a btrfs
+restore directory:
+
+  cd /target_btrfs_path
+  cp /path_to_tape_root/backups/@<server-name>/archive-<subvol-name>/<subvol-id>_full.btrfs .
+  cat <subvol-id>_full.btrfs  | btrfs receive -v  .
+  rm <subvol-id>_full.btrfs
+
+Please consult btrfs-send man-page for further info.
+
 ## Contributing
 
 Help is very welcome! Feel free to fork and issue a pull request to add