improvements to readme

while going through the readme, i noticed a few things that can be improved to make the user experience better

- do not require cloning of the repository for users to try it out so it's easy to get started
- mention`imagePullSecrets` are optional if trying with a public image
- provide a happy example (for example, this repo's image) and a non-happy example
- mention that this requires `actions/attest` and does not work with cosign-signed artifacts that use .sig in registry