[Relays] Document relay setup process for Alpine Linux

Problem to solve

Alpine Linux limits the number of allowed file descriptors for processes started by open-rc, leading to a "Failing because we have 4063 connections already..." warn message.

Proposal

Documentation should include steps to raise this limit by creating an init script containing an rc_ulimit directive, and provide more general steps to setup relays on Alpine Linux.

Who can address the issue

@jsw