Document the MQTT integration

[kevinchalet]

OpenNetty has a MQTT companion (named OpenNetty.Mqtt) that can be enabled using .AddMqttIntegration():

opennetty-core/src/OpenNetty.Mqtt/OpenNettyMqttExtensions.cs

Lines 19 to 48 in 92712cc

	/// <summary>
	/// Registers the OpenNetty MQTT services in the DI container.
	/// </summary>
	/// <param name="builder">The services builder used by OpenNetty to register new services.</param>
	/// <remarks>This extension can be safely called multiple times.</remarks>
	/// <returns>The <see cref="OpenNettyMqttBuilder"/> instance.</returns>
	public static OpenNettyMqttBuilder AddMqttIntegration(this OpenNettyBuilder builder)
	{
	ArgumentNullException.ThrowIfNull(builder);
	builder.Services.AddOptionsWithValidateOnStart<OpenNettyMqttOptions>();
	builder.Services.TryAddSingleton(static provider => new MqttFactory());
	builder.Services.TryAddSingleton(static provider =>
	{
	var factory = provider.GetRequiredService<MqttFactory>();
	return factory.CreateManagedMqttClient();
	});
	builder.Services.TryAddEnumerable(ServiceDescriptor.Singleton<
	IOpenNettyHandler, OpenNettyMqttHostedService>());
	builder.Services.TryAddEnumerable(ServiceDescriptor.Singleton<
	IPostConfigureOptions<OpenNettyMqttOptions>, OpenNettyMqttConfiguration>());
	builder.Services.TryAddSingleton<IOpenNettyMqttWorker, OpenNettyMqttWorker>();
	builder.Services.AddHostedService<OpenNettyMqttHostedService>();
	return new OpenNettyMqttBuilder(builder.Services);
	}

We'll need to document that in the README file.