document Flatpak software installation

admin-guide/configuration/software/packages.md

@@ -87,6 +87,35 @@ To install a packages for specific python versions use the tag `:pip<version>` (
 
 Note that packages installed with `pip` are not updated automatically!
 
+
+## Install Software using Flatpak
+
+The `flatpak` tag can be used to install a software via flatpak. Best use the full Flatpak Application ID, like `pygame-utility`:
+
+```yaml
+base::pkg_group::my_flatpak_software:
+    - 'dev.zed.Zed:flatpak'
+```
+
+On [Flathub](flathub) you find the Application ID if you press the down arrow next to the `Install` button:
+
+![Flathub install commands](packages/zed_on_flathub.png)
+
+Note that flatpak software is by default updated automatically, see [guide "Automated Package Updates"](package_updates) for details.
+
+The Flathub package "remote" is already installed, to add other "remotes" check out the [Configure Flatpak Remotes](flatpak_remotes) guide.
+
+## Remove Packages
+
+To remove an already installed package, append the `:absent` tag to the package name in the package group:
+
+```
+base::pkg_group::java:
+    - 'java-1.8.0-openjdk:absent'
+    - 'java-11-openjdk'
+    - 'java-17-openjdk'
+```
+
 ## Remove Packages
 
 To remove an already installed package, append the `:absent` tag to the package name in the package group: