Mark cdl.el as obsolete

* lisp/obsolete/cdl.el: Add Obsolete-since header.  It is fully
replaceable by shell-command and shell-command-on-region.

etc/NEWS

@@ -1292,6 +1292,10 @@ change the selection rules.
 
 ** Miscellaneous
 
+---
+*** cdl.el is now obsolete.
+Use 'shell-command' and 'shell-command-on-region' instead.
+
 ---
 *** kermit.el is now obsolete.
 

lisp/obsolete/cdl.el

@@ -5,6 +5,7 @@
 ;; Author: Ata Etemadi <ATAE@spva.physics.imperial.ac.uk>
 ;; Maintainer: emacs-devel@gnu.org
 ;; Keywords: data
+;; Obsolete-since: 31.1
 
 ;; This file is part of GNU Emacs.
 
@@ -23,6 +24,10 @@
 
 ;;; Commentary:
 
+;; This library is obsolete.
+;;
+;; Use `shell-command' and `shell-command-on-region' instead.
+
 ;;; Code:
 
 (defun cdl-get-file (filename)