feature request: add a `release-notes` output to the action

[nbusseneau]

Hey, awesome action, been using it for years now 😄

I would find it very useful if the action was providing the rotated release notes as an output (https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#outputs-for-docker-container-and-javascript-actions).

This would allow feeding the release notes right into the GitHub Releases API when creating a new release, and avoid the need to set it up separately.

[thomaseizinger]

Sounds reasonable! PRs welcome :)

[nbusseneau]

Tried something at #28.