Improve Documentation for Input List Format

[developStorm]

trafficstars

[phillip-stephens]

This is a good issue, we should document all the various ways you pass input to ZDNS. A non-exhaustive list:

• Single Domain: echo "censys.io" | ./zdns A
• Single Domain with Name Server: echo "censys.io,1.1.1.1" | ./zdns A
• Multiple Domains: cat domains.txt | ./zdns A

It's unclear what format ZDNS expects input. Let's inventory the ways ZDNS expects input and document them.

At minimum:

• [ ] --help text
• [ ] README.md