add a NEWS symlink for Debian packaging

Quoting [0]:

  If an upstream release notes file is available, containing a summary of
  changes between upstream releases intended for end users of the package and
  often called NEWS, it should be accessible as /usr/share/doc/package/NEWS.gz.
  ...
  If the upstream release notes or changelog do not already conform to this
  naming convention, then this may be achieved either by renaming the files, or
  by adding a symbolic link, at the maintainer’s discretion.

  [0] https://www.debian.org/doc/debian-policy/ch-docs.html#s-changelogs