Changelog: add entry for #15583.

debian/changelog

@@ -58,6 +58,8 @@ tails (3.12) UNRELEASED; urgency=medium
     - Upgrade Enigmail to 2.0.8-5~deb9u1 (Closes: #15657).
     - Upgrade Torbirdy to 0.2.6-1~bpo9+1 (Closes: #15661).
     - Modify Torbirdy configuration in a way that's easier to maintain.
+    - Tell the user they need to use sudo when they attempt to use su
+      (Closes: #15583).
 
   * Build system
     - Make the build of the USB image reproducible (Closes: #15985).