[Doc] Add Custom Metrics Example via Kube-state-metrics

[yashvardhanmishra]

What this PR does / why we need it:

This PR adds a example in the README file to demonstrate how to extend kapp-controller's metrics using kube-state-metrics CustomResourceStateMetrics

Which issue(s) this PR fixes:

Fixes #1159

Does this PR introduce a user-facing change?

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[neil-hickey]

hey @yashvardhanmishra ,thanks for this! It's great, happy to merge. Looks like our bot needs you to sign your commit. Typically done via git commit -s, this is a required for CNCF projects, so let us know if you need any help in doing that!

[neil-hickey]

LGTM when commit is signed, happy to merge