Improve Systemd configuration section in README

[ubitux]

What is the purpose of this change? What does it change?

Elaborate the systemd configuration; useful for casual users and packagers.

Was the change discussed in an issue or in the forum before?

No, personal struggle was the motivator.

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [ ] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [ ] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review