Add README and LICENCE

- Document features, installation, and usage
- Add MIT licence with British English spellings

LICENCE

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+MIT Licence
+
+Copyright (c) 2026
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicence, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# Modbus Slave Simulator
+
+A configurable Modbus slave simulator for testing Modbus TCP, RTU, and TLS connections.
+
+## Features
+
+- **Multiple Protocols**: TCP, RTU (serial), and TLS support
+- **Configurable Data Store**: Customisable sizes for coils, discrete inputs, holding registers, and input registers
+- **Initial Values**: Set default values for all data types
+- **Device Identity**: Configurable vendor name, product code, model, and version
+- **Docker Support**: Ready-to-use Docker image
+
+## Installation
+
+### From Source
+
+```bash
+git clone <repository-url>
+cd modbus-slave
+pip install -r requirements.txt
+```
+
+### With Docker
+
+```bash
+docker pull <username>/modbus-slave
+```
+
+## Usage
+
+### Command Line
+
+```bash
+# TCP server (default: 127.0.0.1:5020)
+python main.py
+
+# Custom port
+python main.py --port 5020
+
+# RTU mode
+python main.py --type rtu --serial-port /dev/ttyUSB0 --slave-id 2
+
+# Custom data store sizes
+python main.py --holding-registers-size 1000 --input-registers-size 500
+
+# Set initial values
+python main.py --holding-registers-default 100 --coils-default true
+```
+
+### Docker
+
+```bash
+# Run with defaults
+docker run -p 5020:5020 <username>/modbus-slave
+
+# Run with custom arguments
+docker run -p 5020:5020 <username>/modbus-slave --holding-registers-size 1000
+```
+
+## Command Line Options
+
+| Option | Description | Default |
+|--------|-------------|---------|
+| `-t, --type` | Server type (tcp, rtu, tls) | tcp |
+| `-h, --host` | TCP bind host | 127.0.0.1 |
+| `-p, --port` | TCP/TLS port | 5020 |
+| `-s, --slave-id` | Modbus slave ID | 1 |
+| `--serial-port` | Serial port (RTU) | /dev/ttyUSB0 |
+| `--baudrate` | Serial baudrate | 9600 |
+| `--coils-size` | Number of coils | 100 |
+| `--holding-registers-size` | Number of holding registers | 100 |
+| `--input-registers-size` | Number of input registers | 100 |
+| `--holding-registers-default` | Default value for holding registers | 0 |
+| `-v, --verbose` | Increase verbosity | - |
+
+## Modbus Data Model
+
+| Function Code | Data Type | Access |
+|--------------|-----------|--------|
+| 0x01/0x05/0x0F | Coil | Read/Write |
+| 0x02 | Discrete Input | Read Only |
+| 0x03/0x06/0x10 | Holding Register | Read/Write |
+| 0x04 | Input Register | Read Only |
+
+## Testing
+
+Test with `pymodbus` client:
+
+```python
+from pymodbus.client import ModbusTcpClient
+
+client = ModbusTcpClient('127.0.0.1', port=5020)
+result = client.read_holding_registers(0, 10)
+print(result.registers)
+client.close()
+```
+
+## Licence
+
+See [LICENCE](LICENCE) file.