Extend documentation

[turion]

I would like to use this action, but there are a few points that are still hard to figure out:

• How/where exactly to add the hackage token in github
• How to make sure that the deployment is only made in specific cases, e.g. when a version tag is added to master

[domenkozar]

See https://github.com/cachix/stamina.hs/blob/main/.github/workflows/publish.yml for inspiration