The lists of new and upgraded packages in 'NEWS' had become way too long
and redundant with what 'guix pull' reports.
* build-aux/update-NEWS.scm (write-packages-added): Don't print ADDED.
(write-packages-updates)[important, table, latest, noteworthy]: New
variables.
Print NOTEWORTHY rather than all of UPGRADED.
(main): Print PREVIOUS-VERSION and NEW-VERSION.
* build-aux/update-NEWS.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
(GUIX_MAINTENANCE_DIRECTORY): New variable.
(update-NEWS): New target.
(.PHONY): Add it.