Improve ZGrab Docs/--help

[phillip-stephens]

Thanks to new contributor @Ashleydai2003 for these suggestions!

Description

• Adds example of using stdin with echo to README.md
• Adds description to zgrab2 --help to explain tool and give guidance on input/output + usage.
• Adds description to http module to give example usage

New --help text:

Usage:
  zgrab2 [OPTIONS] <command>

zgrab2 is fast, modular L7 application-layer scanner. It is commonly used with tools like ZMap which identify "potential services", or services we know are active on a given IP + port, and these are fed into ZGrab2 to confirm and provide details of the service. It has support for a number of protocols listed below as 'Available commands' including SSH and HTTP. By default, zgrab2 will accept input from stdin
and output results to stdout, with updates and logs to stderr. Please see 'zgrab2 <command> --help' for more details on a specific command.
Input is taken from stdin or --input-file, if specified. Input is CSV-formatted with 'IP, Domain, Tag, Port' or simply 'IP' or 'Domain'. See README.md for more details.

Example usages:
echo '1.1.1.1' | zgrab2 tls          # Scan 1.1.1.1 with TLS
echo example.com | zgrab2 http     # Scan example.com with HTTP