Add description for PING command.

[418status]

The PING command lacks a descriptive explanation.

Issue Redis commands

Usage: mini-redis-cli [OPTIONS] <COMMAND>

Commands:
  ping
  get        Get the value of key
  set        Set key to hold the string value
  publish    Publisher to send a message to a specific channel
  subscribe  Subscribe a client to a specific channel or channels
  help       Print this message or the help of the given subcommand(s)

[Darksonn]

Thank you.