Fix binary releases, add RC service example

[Kariton]

• fix cross install for build
• fix freebsd build
• fix automated binary release
• add RC service example (OPNsense) to README.MD

I have no clue how GitHub actions work and how i could make a release in my fork.
But the artifacts are there and working (at least FreeBSD).

fixes #59
closes #91
resolves #117
resolves #118
resolves #119

p.s.: I still have no clue about github actions. but it works :)
see https://github.com/Kariton/prometheus_wireguard_exporter/releases/tag/v3.6.6

[Kariton]

The initially documented "adduser" command created an user with UID > 1000 - and was unable to survive a reboot.
With an ID < 1000 e.g. 518 (needs to be unused!) it works.

i choose 518 because 5180 is the default port for wireguard; there is actually no real reason.
i just tried to find a package (common service user) that already uses this id and there was none

[Kariton]

@MindFlavor i actually managed to get it working.
this will close other PRs as stated.

any chance to get this reviewed and merged?