Update CONTRIBUTING for registry operator

[michael-valdron]

Which area/kind this issue is related to?

/area registry
/area documentation

Issue Description

The information under CONTRIBUTING.md is outdated and needs to be updated to the current state of the registry operator.

Related issue: #1190

Acceptance Criteria

• Replace prerequisites with a reference to the requirements section under the README
• Include install-cert as an required step before install and deploy
• Include docker-buildx as an optional build and push step for multi arch covered in Updates for operator sdk v1.28 registry-operator#45

[michael-valdron]

This has been completed in devfile/registry-operator#46, closing now.