docs: persistent subscription (#577)

docs/en/user-guide/persistent-subscription.md

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+# Persistent and automatically update subscriptions
+
+[systemd.timer](https://www.freedesktop.org/software/systemd/man/latest/systemd.timer.html) allows you to run a service periodically.
+
+The current behavior of dae is to pull subscriptions at startup, but this can sometimes result in an empty group if there is a network anomaly or other possible reason when pulling subscriptions.
+
+We'll introduce a way to achieve persistent subscription storage and automatic subscription updates through the systemd service and timer.
+
+## Preparations
+
+### Shell script
+
+We assume that your dae configuration file is stored in `/usr/local/etc/dae/` .
+
+`/usr/local/bin/update-dae-subs.sh`:
+
+```sh
+#!/bin/bash
+
+# Change the path to suit your needs
+cd /usr/local/etc/dae || exit 1
+version="$(dae --version | head -n 1 | sed 's/dae version //')"
+UA="dae/${version} (like v2rayA/1.0 WebRequestHelper) (like v2rayN/1.0 WebRequestHelper)"
+while IFS=':' read -r name url
+do
+    curl -fL -A "$UA" "$url" -o "${name}.sub.new"
+    if [[ $? -eq 0 ]]; then
+        mv "${name}.sub.new" "${name}.sub"
+        chmod 0600 "${name}.sub"
+        echo "Downloaded $name"
+    else
+        rm "${name}.sub.new"
+        echo "Failed to download $name"
+    fi
+done < sublist
+
+dae reload
+```
+
+You need to give it proper permission:
+```sh
+chmod +x /usr/local/bin/update-dae-subs.sh
+```
+
+### `systemd.timer` and `systemd.service`
+
+This timer will automatically update dae subscriptions every 12 hours, or 15 minutes after each boot.
+
+`/etc/systemd/system/update-subs.timer`:
+
+```systemd
+[Unit]
+Description=Auto-update dae subscriptions
+
+[Timer]
+OnBootSec=15min
+OnUnitActiveSec=12h
+
+[Install]
+WantedBy=timers.target
+```
+
+`/etc/systemd/system/update-subs.service`:
+
+```systemd
+[Unit]
+Description=Update dae subscriptions
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/update-dae-subs.sh
+```
+
+## Configurations
+
+Put your subscription links into `/usr/local/etc/dae/sublist`:
+
+```text
+sub1:https://mysub1.com
+sub2:https://mysub2.com
+sub3:https://mysub3.com
+```
+
+Give the file appropriate permissions (for your privacy):
+
+```sh
+chmod 0600 /usr/local/etc/dae/sublist
+```
+
+Edit `config.dae`:
+
+```text
+subscription {
+    # Add your subscription links here.
+    sub1:'file://sub1.sub'
+    sub2:'file://sub2.sub'
+    sub3:'file://sub3.sub'
+}
+```
+
+## Enable timer
+
+Execute the following command:
+
+```sh
+systemctl enable --now update-dae-subs.timer
+
+# If you need to renew your subscription immediately or haven't pulled a subscription before
+systemctl start update-dae-subs.service
+```