chore: add docs (everything is in one file)

Author: rodriguezdamien

topics/p2p-transfer-protocol/docs/README.md

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+# P2P Transfer Protocol (Peerile)
+## What's this?
+This is a command-line tool for transferring files between two machines on the same network, without a central server. The tool can function as both sender and receiver.
+
+## Architecture
+### Project Structure
+The project is organized into 4 modules:
+- `protocol.rs`: Communication protocol definition, where all protocol commands are declared and defined for "peer-to-peer" communication.
+- `listener.rs`: File reception management, it listens for incoming connections and handle them.
+- `sender.rs`: File sending management, it tries to connect to a listener in order to send a file.
+- `main.rs`: Entry point & command-line interface declaration
+
+### Communication Protocol
+The protocol uses four commands:
+1. **HELLO filename size**: The sender proposes a file to the receiver
+  - `filename`: name of the file to transfer
+  - `size`: file size in bytes
+2. **ACK**: The receiver accepts the proposed file
+3. **NACK**: The receiver refuses the proposed file
+4. **SEND size**: The sender starts sending the file
+  - `size`: must match the size announced in HELLO
+
+All commands end with a newline character (`\n`) which serves as a delimiter for the program.
+
+## How it works?
+### Receiver Side (listener)
+1. The receiver listens on a specified port (or by default 9000)
+2. For each incoming connection, a new thread is created
+3. The receiver waits for a HELLO command indicating a filename and its size
+4. If the file doesn't already exist, it sends ACK, otherwise NACK
+5. It waits for the SEND command with the corresponding size
+6. It receives the file data and writes it to the output directory
+
+### Sender Side
+1. The sender connects to the receiver's IP address and port
+2. It sends a HELLO command with the file name and size
+3. It waits for the receiver's response (ACK or NACK)
+4. If ACK is received, it sends the SEND command followed by the file data
+5. If NACK is received, the transfer is cancelled
+
+## Usage
+### Receiving a File
+> Command examples use `peerile`, but you can use `cargo run --` instead. (the `--` is important!)
+```bash
+peerile listen --port 9000 --output ./shared
+```
+- `--port | -p`: Port on which the program will listen for incoming file transfers
+- `--output | -o`: destination for received files
+
+### Sending a File
+
+```bash
+peerile send --file document.pdf --to 192.168.1.100 --port 9000
+```
+
+- `--file | -f`: Path to the file to send
+- `--to | -t`: Receiver's IP address
+- `--port | -p`: Port on which the receiver is listening
+