add release notes to published RC releases

[shiftkey]

I'd love to consume pre-release packages in dugite-native but releases like v2.20.0-rc0.windows.1 do not contain the checksum details for the assets in the release (which I use to verify the integrity of bits when packaging).

I'm less interested in the release notes themselves for RC packages (maybe they're still being worked on at that point, but I see your Git mailing list announcement has them too?).

[dscho]

Seems to have worked: https://github.com/git-for-windows/git/releases/tag/v2.20.0-rc1.windows.1