Clarify itch.io release workflow in README

[alice-i-cecile]

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 #13 (comment)