Add instructions on how to run with systemd

[swissrouting]

Description

Based on the instructions I already wrote up on https://www.swissrouting.com/nostr.html#installing-as-a-service

Related Issue

Related: issue #44

Motivation and Context

By default the server doesn't restart on system reboot. These instructions provide a way to solve that.

How Has This Been Tested?

• Run on my own system

Types of changes

• Non-functional change (docs, style, minor refactor)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my code changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 3768540257

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.682%

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Totals
Change from base Build 3765539114:	0.0%
Covered Lines:	827
Relevant Lines:	932

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💛 - Coveralls

[github-actions]

🎉 This PR is included in version 1.12.0 🎉

The release is available on GitHub release

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