Add documentation about automatic client updater

doc/extrapkg.md

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+# Extra_pkg
+
+Installs additional packages from stable or backports repositories.
+
+## Configuration / Variables
+
+* **extra_pkg**  
+  List of packages from the stable repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+* **extra_pkg1**  
+  List of packages from the stable repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+* **extra_pkg2**  
+  List of packages from the stable repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+* **extra_pkg_bpo**  
+  List of packages from the backports repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+* **extra_pkg_bpo1**  
+  List of packages from the backports repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+* **extra_pkg_bpo2**  
+  List of packages from the backports repository to be installed.  
+  Type: **List of Strings**  
+  Default: `[]`
+
+## Example
+
+All hosts receive the following packages:
+* kicad
+* freecad
+
+Devices in the teacherlaptops group additionally receive:
+* plasma-discover
+* nextcloud-desktop
+* dolphin-nextcloud
+
+```yaml
+inventory.yml
+all:
+  vars:
+    extra_pkg:
+      - kicad
+      - freecad
+teacherlaptops:
+  hosts:
+    - nbt[001:999]:
+  vars:
+    extra_pkg1:
+      - plasma-discover
+      - nextcloud-desktop
+      - dolphin-nextcloud
+```