message: add "manifest" into the readme
diff: -Flatpak can be found at: https://github.com/flathub/com.github.Murmele.Gittyup
  +Flatpak manifest can be found at: https://github.com/flathub/com.github.Murmele.Gittyup
  Arch PKGBUILD file can be found at: TODO (currently it is in #314)
  