Add --clean-download-directory to README.md

[hashar]

The option has been added via #239 in 2020. Add it to the README.md for those reading the documentation from the GitHub repo page.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[hashar]

The reason for this pull request is --clean-download-directory is not showing up at https://github.com/jenkinsci/plugin-installation-manager-tool/#cli-options :)

[MarkEWaite]

Thanks!