docs: add "Run as Linux Service via Systemd" to README

[windix]

I have recently added addguardhome-sync daemon as a service via systemd.

Thought it would be helpful to have a section in README for others to refer to.

[coveralls]

Pull Request Test Coverage Report for Build 9542197002

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 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 63.077%

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Totals
Change from base Build 9494330855:	0.0%
Covered Lines:	1230
Relevant Lines:	1950

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💛 - Coveralls

[bakito]

Hi @windix
Thank you very much for this contribution.