Update README with tool description

Added a brief description of the tool's functionality.

Author: PRProd

README.md

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 ![Rust](https://img.shields.io/badge/rust-1.71+-orange.svg)
 [![License: GPL v3](https://img.shields.io/badge/License-GPL%20v3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![Rust](https://github.com/PRProd/dreamhost-ddns/actions/workflows/rust.yml/badge.svg)](https://github.com/PRProd/dreamhost-ddns/actions/workflows/rust.yml)
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 A lightweight Rust CLI tool that updates a DreamHost DNS **A record** with your current public IP address.
 
 This tool is designed for: