Clarify itch.io release workflow in README

I think a new section `### Publish on GitHub release` could be nice and mirror the new `### Publish on itch.io`
(and would be easy to expand if we add more targets later, an easy one would be a `publish to GitHub page` with a custom html and/or the wasm version for example)

_Originally posted by @mockersf in https://github.com/bevyengine/bevy_github_ci_template/pull/13#discussion_r964292082_