3.5.1.2

OJSBR build: packaging, release automation and version bump

- README describing this as an unofficial build of ojsde/shariff stable-3_5_0
  plus the commits proposed in ojsde/shariff#54; the original README is kept
  as README-upstream.md
- .github/workflows/release.yml and ghcr.yml to publish the installable
  tarball on tag push
- .gitattributes/.gitignore so the package stays clean
- version.xml bumped to 3.5.1.2 so OJS accepts the upload over an existing
  3.5.1.1 install