add CITATION.cff file

As per spec: https://citation-file-format.github.io/

Fixes: #35260

CITATION.cff

@@ -0,0 +1,15 @@
+cff-version: 1.2.0
+title: systemd
+type: software
+repository-code: 'https://github.com/systemd/systemd'
+url: 'https://systemd.io/'
+abstract: >-
+  systemd is a suite of basic building blocks for a Linux
+  system. It provides a system and service manager that runs
+  as PID 1 and starts the rest of the system.
+keywords:
+  - Linux
+  - PID 1
+  - Service Manager
+  - Boot
+license: LGPL-2.1-or-later