Write better documentation

[zhoreeq]

Explain -useconffile flag usage and how it works without a config.

More detailed dnsmasq configuration description. How to set it as a system resolver.

Explain systemd file customization.

Standalone mode must be explained better.

[marek22k]

Explain -useconffile flag usage and how it works without a config.

This option does not seems to exist:

$./meshnamed -useconffile test.conf
flag provided but not defined: -useconffile
Usage of ./meshnamed:
  -debug
    	enable debug logging
  -getip string
    	convert a name to IPv6 address
  -getname string
    	convert IPv6 address to a name
  -listenaddr string
    	address to listen on (default "[::1]:53535")
  -networks string
    	TLD=subnet list separated by comma (default "ygg=200::/7,cjd=fc00::/8,meshname=::/0,popura=::/0")
  -nomeship
    	disable .meship resolver