Add systemd configuration in documentation (#2605)

See #2561 and #2557

docs/en/admins/08_FeedUpdates.md

@@ -1,14 +1,18 @@
 # Setting Up Automatic Feed Updating
 
-FreshRSS updating is controlled by a script located at `./app/actualize_script.php`. Knowing this, we can create a Cron job to launch the update script.
+FreshRSS is updated by the `./app/actualize_script.php` script. Knowing this, we can periodically trigger it to ensure up-to-date feeds.
 
-**Note:** the update script will not fetch feeds more often than every twenty minutes, so there's no sense in setting the Cron job to run anymore often than that.
+**Note:** the update script won't update any particular feed more often than once every twenty minutes, so it doesn't make sense to trigger it much more frequently than that.
 
-You will need to check the Cron documentation for your specific distribution ([Debian/Ubuntu](https://help.ubuntu.com/community/CronHowto), [Red Hat/Fedora/CentOS](https://fedoraproject.org/wiki/Administration_Guide_Draft/Cron), [Slackware](https://docs.slackware.com/fr:slackbook:process_control?#cron), [Gentoo](https://wiki.gentoo.org/wiki/Cron), [Arch Linux](https://wiki.archlinux.org/index.php/Cron)...) to insure you set the Cron job correctly.
+**Note:** the following examples assume that FreshRSS is installed to `/usr/share/FreshRSS`. You'll need to modify the FreshRSS path to reflect your own system.
+
+## Cron as a trigger
+
+You'll need to check the Cron documentation for your specific distribution ([Debian/Ubuntu](https://help.ubuntu.com/community/CronHowto), [Red Hat/Fedora/CentOS](https://fedoraproject.org/wiki/Administration_Guide_Draft/Cron), [Slackware](https://docs.slackware.com/fr:slackbook:process_control?#cron), [Gentoo](https://wiki.gentoo.org/wiki/Cron), [Arch Linux](https://wiki.archlinux.org/index.php/Cron) ...) to make sure you set the Cron job correctly.
 
 It's advisable that you run the Cron job as your Web server user (often `www-data`).
 
-## Example on Debian/Ubuntu
+### Example on Debian/Ubuntu
 To run the updater script every hour, and 10 minutes past the hour:
 
 Run `sudo crontab -e` and copy the following line into the crontab:
@@ -16,4 +20,37 @@ Run `sudo crontab -e` and copy the following line into the crontab:
 10 * * * * www-data php -f /usr/share/FreshRSS/app/actualize_script.php > /tmp/FreshRSS.log 2>&1
 ```
 
-This crontab example, of course, assumes that FreshRSS is installed in `/usr/share/FreshRSS`; if you've installed it somewhere else, be sure to correct the path in your crontab entry.
+## Systemd as a trigger
+
+Some systems can't use a Cron job, but they can use systemd. It's easy to configure it to mimic Cron's features.
+
+First you need to add a `freshrss.timer` file in `/etc/systemd/system/` with the following content:
+
+```
+[Unit]
+Description=FreshRSS get new content
+
+[Timer]
+OnBootSec=30s
+OnCalendar=*:0/20
+
+[Install]
+WantedBy=timers.target
+```
+
+This timer will start 30 seconds after boot and it will trigger the service every 20 minutes. Feel free to change the configuration to better suit your needs.
+
+Then you need to add a `freshrss.service` file in the same directory. This will be the description of the service triggered by the aforementioned timer.
+
+```
+[Unit]
+Description=FreshRSS get new content
+Wants=freshrss.timer
+
+[Service]
+User=www-data
+Type=simple
+ExecStart=/usr/bin/php /usr/share/FreshRSS/app/actualize_script.php
+```
+
+Finally, you need to enable the timer with `systemctl enable freshrss.timer` and reload the systemd configuration with `systemctl daeamon-reload`.