Add demo/diff to top of readme

[jdolitsky]

I've been adding some demo steps and diff link to the top of the README for repos in oci-playground:

• https://github.com/oci-playground/zot#oci-playground
• https://github.com/oci-playground/go-containerregistry#oci-playground

something similar to

# OCI Playground

[View diff](https://github.com/project-zot/zot/compare/main...oci-playground:main)

To build:
\```
make binary
\```

To run:
\```
rm -rf /tmp/zot

bin/zot-* serve examples/config-conformance.json
\```

To test:
\```
curl -v http://localhost:8080/v2/myrepo/references/sha256:f6d0597d725e89075c9010d63fcbe4eb0b36ac15db9cfa504695ee2b17940eba
\```