声明脚本的用法

参考 mirrord 的安装脚本如下：

#!/bin/bash
# mirrord installer
#             _                        _ 
#   _ __ ___ (_)_ __ _ __ ___  _ __ __| |
#  | '_ ` _ \| | '__| '__/ _ \| '__/ _` |
#  | | | | | | | |  | | | (_) | | | (_| |
#  |_| |_| |_|_|_|  |_|  \___/|_|  \__,_|
#
# Usage:
#   curl -fsSL https://github.com/metalbear-co/mirrord/raw/latest/scripts/install.sh | sh