v0.4.0

Changed

• Renamed the build tasks away from the registry-derived push/pull
  verbs, which were misleading for a tarball-over-SSH workflow (nothing is
  pushed to a registry, and "pull" actually uploaded to the server):
  • mix xamal.build.push → mix xamal.build (build the tarball locally)
  • mix xamal.build.pull → mix xamal.build.upload (upload the tarball to servers)
  • mix xamal.build.deliver and mix xamal.build.details are unchanged.
  • The --skip-push deploy option is renamed to --skip-build (it skips the
    build and uploads an existing tarball). These are hard renames with no
    deprecation aliases.

Added

• CONTRIBUTING.md documenting the development workflow, the ## [Unreleased]
  changelog convention, and the maintainer release process.