Add a CHANGELOG / git tag for published versions

[rnestler]

Currently it's a bit hard to figure out what changed between releases on crates.io. An easy thing to improve would be to add git tags per version published to crates.io, providing a CHANGELOG (with a format like https://keepachangelog.com/) is probably more effort.

[Morganamilo]

I guess that's probably a good idea now that it's an archlinux project and has some use.

[Morganamilo]

Pushed tags for alpm and alpm-sys. Not sure about alpm-utils has I've been thinking of moving that to a separate git.