doc: improve rollup readme

[icemelon]

Purpose or design rationale of this PR

improve rollup readme

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• docs: Documentation-only changes

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• No, this PR doesn't involve a new deployment, git tag, docker image tag
• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change
• Yes

[codecov]

Codecov Report

Merging #917 (4df4c0e) into develop (d3c2e34) will not change coverage.
Report is 2 commits behind head on develop.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #917   +/-   ##
========================================
  Coverage    49.27%   49.27%           
========================================
  Files           95       95           
  Lines         9888     9888           
========================================
  Hits          4872     4872           
  Misses        4612     4612           
  Partials       404      404           

Flag	Coverage Δ
rollup	62.35% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.