improvements to readme

[sozercan]

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
• mentionimagePullSecrets 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