add systemd service

Fixes #2.

add timer; make systemd unit instantiated

add some information on systemd unit

README.md

@@ -5,19 +5,45 @@
 This bash script sends incremental snapshots to another drive for backing up
 data. Plug in and mount any btrfs-formatted device you want your system to be
 backed up to (like a USB drive).  When you run the script you will be prompted
-to select a mounted btrfs device.
+to select a mounted btrfs device, or you can optionally select the disk using
+its UUID on the command line.
 
-The script iterates through all snapper configurations. For each configuration
-it creates a new local snapshot. If you have never synced to the specified
-device you will be prompted to enter a directory on the device where the backup
-snapshots will go. Additionally you are shown the location of the backed up
-snapshot. If you have performed a backup to this device before, only the changes
-since the last backup have to be sent.
+The script iterates through all snapper configurations by default (this can be
+changed using the `-c` flag). For each configuration it creates a new local
+snapshot. If you have never synced to the specified device you will be prompted
+to enter a directory on the device where the backup snapshots will go.
+Additionally you are shown the location of the backed up snapshot. If you have
+performed a backup to this device before, only the changes since the last backup
+have to be sent.
 
 ## Requirements
 
 snapper is required.
 
+
+## Options
+
+	Usage: snap-sync [options]
+
+	Options:
+	 -d, --description <desc> Change the snapper description. Default: "latest incremental backup"
+	 -c, --config <config>    Specify the snapper configuration to use. Otherwise will perform for each snapper
+							  configuration. Can list multiple configurations within quotes, space-separated
+							  (e.g. -c "root home").
+	 -n, --noconfirm          Do not ask for confirmation for each configuration. Will still prompt for backup
+							  directory name on first backup
+	 -u, --UUID <UUID>        Specify the UUID of the mounted BTRFS subvolume to back up to. Otherwise will prompt.
+							  If multiple mount points are found with the same UUID, will prompt user.
+
+## Systemd unit and timer
+
+A systemd unit and timer are included. These are instantiated units. You need to
+specify the UUID of the disk to back up to in the `systemctl` call. Note, once
+again, the disk **must** be mounted, and it **must** only be mounted in one
+place. Example:
+
+    # systemctl start snap-sync@7360922b-c916-4d9f-a670-67fe0b91143c
+
 ## Example output
 
     # snap-sync