Add link to complete list of keys

Guides/os.md

@@ -48,6 +48,8 @@ Set your _desktop_ file in `~/.local/share/applications/`.
 
 [Tutorial to create a desktop file](https://linuxconfig.org/how-to-create-custom-desktop-files-for-launchers-on-linux).
 
+[Complete list of keys on freedesktop.org](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#recognized-keys).
+
 ## Grub
 
 `sudo update-grub`