docs: Mention systemd's ReadWritePaths setting for the backup dir

The systemd unit file uses `ProtectSystem=strict`, which makes almost
every directory read-only. This can cause backups to not work, even if
the directory is granted the correct permissions and ownership to the
`conduwuit` user.

The `ReadWritePaths` setting lets you specify which directories are
exempt from being made read-only by `ProtectSystem=strict`.

conduwuit-example.toml

@@ -95,6 +95,10 @@
 # engine API. To use this, set a database backup path that continuwuity
 # can write to.
 #
+# If you are using systemd, you will need to add the path to
+# ReadWritePaths in the service file, preferably via a drop-in file
+# through `systemctl edit`.
+#
 # For more information, see:
 # https://continuwuity.org/maintenance.html#backups
 #

src/core/config/mod.rs

@@ -145,6 +145,10 @@ pub struct Config {
 	/// engine API. To use this, set a database backup path that continuwuity
 	/// can write to.
 	///
+	/// If you are using systemd, you will need to add the path to
+	/// ReadWritePaths in the service file, preferably via a drop-in file
+	/// through `systemctl edit`.
+	///
 	/// For more information, see:
 	/// https://continuwuity.org/maintenance.html#backups
 	///