generic-linux-gpu: add a news entry for GNU Shepherd support

Also encourage contribution for other init systems.

Author: gs-101

modules/misc/news/2025/12/2025-12-22_16-55-56.nix

@@ -0,0 +1,13 @@
+{ pkgs, ... }:
+
+{
+  time = "2025-12-22T19:55:56+00:00";
+  condition = pkgs.stdenv.hostPlatform.isLinux;
+  message = ''
+    `targets.genericLinux.gpu` now supports GNU Shepherd.
+
+    This module now supports the GNU Shepherd init system, primarily for
+    compatibility with the GNU Guix distributuion. This change also generally
+    facilitated the implementation of other init systems.
+    '';
+}