rl-2003: add note about slim

Author: worldofpeace

nixos/doc/manual/release-notes/rl-2003.xml

@@ -131,6 +131,13 @@
       <link linkend="opt-networking.interfaces">networking.interfaces.&lt;name&gt;.…</link> options.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The SLIM Display Manager has been removed, as it has been unmaintained since 2013.
+     Consider migrating to a different display manager such as LightDM (current default in NixOS),
+     SDDM, GDM, or using the startx module which uses Xinitrc.
+     </para>
+   </listitem>
   </itemizedlist>
  </section>