docs: added linux systemd setup configuration

docs/systemd_details.md

@@ -10,4 +10,73 @@ To control the tmux service you can use all the standard `systemctl` commands us
 
   systemctl --user status tmux.service
 
+## Setup Example
 
+To be able to configure systemd user services, make sure the following directories exist: `$HOME/.config/systemd/user`. If not, just create them:
+
+```shell
+mkdir ~/.config/systemd
+mkdir ~/.config/systemd/user
+```
+
+Create the systemd user service file:
+
+```shell
+touch ~/.config/systemd/user/tmux.service
+```
+
+The service file can be created from this template:
+
+```shell
+[Unit]
+Description=tmux default session (detached)
+Documentation=man:tmux(1)
+After=graphical.target
+
+[Service]
+Type=forking
+Environment=DISPLAY=:1
+ExecStart=/usr/bin/tmux new-session -d
+
+ExecStop=/home/user/.tmux/plugins/tmux-resurrect/scripts/save.sh
+ExecStop=/usr/bin/tmux kill-server
+KillMode=mixed
+
+RestartSec=2
+
+[Install]
+WantedBy=default.target
+```
+
+Now make sure you adapt the service file to your needs:
+
+- `Environment`: Set the value of your $DISPLAY environment variable (i.e. `:1`, to find out run `echo $DISPLAY`)
+- `ExecStart`: If you want to configure the tmux start command, you can do it here
+- `ExecStop`: Enter the full path to the `save.sh` script of `tmux-resurrect`, usually in `$HOME/.tmux/plugins/tmux-resurrect/scripts/save.sh`
+- `After`: Adapt to your needs, waiting for `graphical.target` helps if you want to open gui applications such as `code` directly from your resurrected terminals
+
+Now you are ready to enable and start your system service:
+
+```shell
+systemctl --user enable tmux.service
+systemctl --user start tmux.service
+```
+
+- Reboot your machine.
+- To check the current status of your tmux service, run this command:
+
+```shell
+systemctl --user status tmux.service
+```
+
+You should see something along the lines of:
+
+```shell
+Active: active (running) since Fri 2022-05-27 15:28:36 CEST; 23h ago
+  Docs: man:tmux(1)
+  Process: 6300 ExecStart=/usr/bin/tmux new-session -d (code=exited, status=0/SUCCESS)
+  CGroup: /user.slice/user-1000.slice/user@1000.service/app.slice/tmux.service
+          ├─   6306 /usr/bin/tmux new-session -d
+          ├─   7735 zsh
+  ...
+```