Improves systemd.path unit (#139)

* grub-btrfs.path : improves unit

#### Remember to modify your configuration accordingly.  
Allows the unit to start and stop automatically when the mount point is detected.  
If the mount point is already mounted when the unit is activated `systemctl enable grub-btrfs.path`,  
it will be necessary to start the unit for monitoring to begin `systemctl start grub-btrfs.path`.

- `DefaultDependencies=no`
Prevents systemd from automatically generating `Wants= or Requires= or After=`.
- `Requires=\x2esnapshots.mount`
Ensures unit is started if mount point exists.
- `After=\x2esnapshots.mount`
Unit start after the mounting point exists.
- `BindsTo=\x2esnapshots.mount`
If the mount point is removed, the unit will stop.
- `PathModified=\x2esnapshots.mount`
The monitored folder containing the snapshots.
- `WantedBy=\x2esnapshots.mount`
If the mount point exists, unit start automatically. (only if unit is activated before the mount point is mounted)

* Readme: Update "Automatically update grub" section

Update section following the recent change in the `grub-btrfs.path` unit.  
Add information to be able to update the `grub-btrfs` menu every time the system is restart/shutdown.

README.md

@@ -6,10 +6,10 @@
 This is a version 4.xx of grub-btrfs
 ##### BTC donation address: `1Lbvz244WA8xbpHek9W2Y12cakM6rDe5Rt`
 ##
-### Description
+### Description :
 Improves Grub by adding "btrfs snapshots" to the Grub menu.
 
-You can start your system on a "snapshot" from the Grub menu.  
+You can boot your system on a "snapshot" from the Grub menu.  
 Supports manual snapshots, snapper, timeshift ...
 
 ##### Warning: booting on read-only snapshots can be tricky
@@ -46,29 +46,52 @@ NOTE: Generate your Grub menu after installation for the changes to take effect.
 On Arch Linux use `grub-mkconfig -o /boot/grub/grub.cfg`.
 
 ##
-### Customization:
+### Customization :
 
 You have the possibility to modify many parameters in `/etc/default/grub-btrfs/config`.  
 See [config file](https://github.com/Antynea/grub-btrfs/blob/master/config) for more information.
 
 ##
 ### Automatically update grub
-If you would like Grub to automatically update when a snapshot is made or deleted:
-* Use `systemctl start/enable grub-btrfs.path`.
-* `grub-btrfs.path` automatically (re)generates `grub.cfg` when a modification appears in `/.snapshots` folder (by default).
-* If your snapshots aren't mounted in `/.snapshots`, you must modify the watch folder using `systemctl edit grub-btrfs.path`.  
-	* For example: Timeshift mount its snapshots in `/run/timeshift/backup/timeshift-btrfs/snapshots` folder.
-
-		Use `systemctl edit grub-btrfs.path`.
-		Then wrote:
+1- If you would like grub-btrfs menu to automatically update when a snapshot is created or deleted:
+* Use `systemctl enable grub-btrfs.path`.
+  * `grub-btrfs.path` automatically (re)generates `grub-btrfs.cfg` when a modification appears in `/.snapshots` mount point (by default).
+  * If the `/.snapshots` mount point is already mounted, then use `systemctl start grub-btrfs.path` to start monitoring.  
+    Otherwise, the unit will automatically start monitoring when the mount point will be available.
+* If your snapshots location aren't mounted in `/.snapshots`, you must modify `grub-btrfs.path` unit using  
+`systemctl edit --full grub-btrfs.path` and run `systemctl reenable grub-btrfs.path` for changes take effect.  
+To find out the name of the `.mount` unit  
+use `systemctl list-units -t mount`.  
+	* For example: Timeshift mounts its snapshot folder in `/run/timeshift/backup/timeshift-btrfs/snapshots`.
+
+		Use `systemctl edit --full grub-btrfs.path`.
+		Then replace the whole block by:
 		```
+		[Unit]
+		Description=Monitors for new snapshots
+		DefaultDependencies=no
+		Requires=run-timeshift-backup.mount
+		After=run-timeshift-backup.mount
+		BindsTo=run-timeshift-backup.mount
+
 		[Path]
 		PathModified=/run/timeshift/backup/timeshift-btrfs/snapshots
+
+		[Install]
+		WantedBy=run-timeshift-backup.mount
 		```
-		and finally save.
+		Then save and finally run `systemctl reenable grub-btrfs.path` for changes take effect.  
+		Optional:  
+		If the `/run/timeshift/backup/timeshift-btrfs/snapshots` mount point is already mounted,  
+		then use `systemctl start grub-btrfs.path` to start monitoring.  
+		Otherwise, the unit will automatically start monitoring when the mount point will be available.  
 	* You can view your change to `systemctl cat grub-btrfs.path`.
 	* To revert change use `systemctl revert grub-btrfs.path`.
-##### Warning:
+
+2- If you would like grub-btrfs menu to automatically update on system restart/shutdown:  
+[Look at this comment](https://github.com/Antynea/grub-btrfs/issues/138#issuecomment-766918328)  
+Currently not implemented  
+##### Warning :
 by default, `grub-mkconfig` command is used.  
 Might be `grub2-mkconfig` on some systems (Fedora ...).   
 Edit `GRUB_BTRFS_MKCONFIG` variable in `/etc/default/grub-btrfs/config` file to reflect this.

grub-btrfs.path

@@ -1,8 +1,12 @@
 [Unit]
 Description=Monitors for new snapshots
+DefaultDependencies=no
+Requires=\x2esnapshots.mount
+After=\x2esnapshots.mount
+BindsTo=\x2esnapshots.mount
 
 [Path]
 PathModified=/.snapshots
 
 [Install]
-WantedBy=multi-user.target
+WantedBy=\x2esnapshots.mount