* Packaging hints for ruby-mdl to Debian

- Because file d/watch monitors https://www.github.com/markdownlint/markdownlint
  as the upstream reference, d/copyright explicitly excludes upstream files and
  folders considered irrelevant to packaging to Debian.  Thus, the archive added
  to the branch `pristine-tar` is repackaged.  As described in man debian-watch,
  `+ds` in the version string indicates this difference.
  The packaging of `sqlitestudio` serves as example of implementation.  This
  includes the use of tar.xz archive instead of tar.gz (current default when
  building is initiated by `gem2deb`).