chore: add README

README.md

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+# Aiken
+
+## Contents
+
+- [About](#about)
+- [How it works](#how-it-works)
+- [Usage](#usage)
+
+## About
+
+Aiken (_jp._ 合件, _lit._ "common subject") is an utility to share files across your LAN. For example, when you are at work and need to send a file to your colleagues quickly.
+
+## How it works
+
+Server wants to share file, client wants to get it. Firstly, they need to find each other in the LAN. The server generates a token needed to get the file. The client transceives a broadcast packet containing this token to find the server. The server checks the token and sends back a response `OK` or `NO` depending on whether token is valid or not. If token is valid, the server sets up a TCP server for the client to send the file. When got `OK` response, the client connects to the server and gets the file.
+
+NOTE: currently you should have only one server in your LAN at one time, because when a client sends broadcasts a token, he can receive a `NO` response from a wrong server before `OK` response from the right one, rendering himself unable to get the file.
+
+## Usage
+
+```
+Usage:
+  aiken <options> [arguments]
+
+Options:
+  -h           Print this help and exit
+  -s <path>    Share the file of the specified <path>
+  -g <token>   Get the file by its token
+  -o <path>    Specify output path when getting file
+               (defaults to current working directory and
+               the initial file name)
+  -V           Print version info and exit
+```
+
+---
+
+_&copy; Masahiko AMANO aka H1K0, 2024-present_