Documentation update

[kasperg]

Thanks for sharing this action. I have setup this action in our own workflow and it works as advertised.

As far as I can tell the instructions in the README are not entirely up to date. They refer to the repo being private, which is no longer the case, mentions steps like setup-node which does not seem to be necessary etc. There may also be an issue with compression and versioning as mentioned in #169

Are you interested in a PR with updates to address these issues?