add release workflow

[g-plane]

This PR adds a GitHub Actions workflow for publishing crate to crates.io.

Once merged this PR, remember to :

1. Go to crates.io and generate a token.
2. Open repository settings on GitHub.
3. Select the "Actions" under the "Secrets and variables":
   
4. Create a repository secret called CARGO_REGISTRY_TOKEN and paste the token into there.

To publish a new version to crates.io, just update the version field of Cargo.toml and create a corresponding Git tag.

Hope it helps. I'm looking forward to new versions of Salsa.

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[MichaReiser]

Thanks; this is great. Would you mind extending the README with a Contributing section that explains how to make a release?

[g-plane]

Done.

[nikomatsakis]

Ooh, this is awesome, thanks!

[nikomatsakis]

Thanks for this. It won't work as is though because publishing salsa requires publishing both salsa and salsa-macros. Also, the two step process (edit version(s), issue git tag) is maybe a bit more complex than I'd like.

My suggestion: let's create a cargo xtask like cargo publish-salsa and then modify it so that...

• cargo publish-salsa with no arguments dumps some help
• cargo publish-salsa prepare modifies Cargo.toml with new version and creates the git tag. It can also check that nothing is dirty in git. We should probably start maintaining release notes but that can wait.
• cargo publish-salsa from-ci actually does the work. Ideally it should also assert that the tag has been pushed to main so we avoid publishing things that don't exist in main. But that can be a FIXME.

[g-plane]

Are they always published in same versions?

You can also check cargo-release, so we don't need to create our own xtask.

[nikomatsakis]

They are always in lockstep. Also I think there are now 3 crates.

https://github.com/release-plz/release-plz also looks neat -- probably there exist utilities for this by now.