[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
issue