Add Pnp convention helper class (#184)

- Update README with instructions on how to use it.

README.md

@@ -371,6 +371,34 @@ if(!res)
 }
 ```
 
+In case a [DPS model](https://github.com/Azure/opendigitaltwins-dtdl/blob/master/DTDL/v2/dtdlv2.md) is going to be used, the ID of the model has to be passed to the ProvisioningDeviceClient and DeviceClient constructor.
+The code above requires the following changes.
+
+Add the model ID as a constant:
+
+```csharp
+public const string ModelId = "dtmi:orgpal:palthree:palthree_demo_0;1";
+
+```
+
+Create the additional payload information with the model ID to be sent along the registration with DPS and pass that to the call to `Register()`.
+
+```csharp
+var pnpPayload = new ProvisioningRegistrationAdditionalData
+{
+    JsonData = PnpConvention.CreateDpsPayload(ModelId),
+};
+
+var myDevice = provisioning.Register(pnpPayload, new CancellationTokenSource(60000).Token);
+
+```
+
+Create the device client passing the model ID to the respective parameter in the constructor.
+
+```csharp
+var device = new DeviceClient(myDevice.AssignedHub, myDevice.DeviceId, SasKey, nanoFramework.M2Mqtt.Messages.MqttQoSLevel.AtLeastOnce, azureCA, ModelId);
+```
+
 Note: like for the `DeviceClient` you need to make sure you are connected to a network properly and also have a proper data and time set on the device.
 
 ### Provisioning using certificates

nanoFramework.Azure.Devices.Client/Azure.Devices.Client.nfproj

@@ -36,6 +36,7 @@
     <Compile Include="IoTHubStatus.cs" />
     <Compile Include="MethodCallback.cs" />
     <Compile Include="DeviceClient.cs" />
+    <Compile Include="PlugAndPlay\PnpConvention.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="SHA256.cs" />
     <Compile Include="PropertyAcknowledge.cs" />

nanoFramework.Azure.Devices.Client/PlugAndPlay/PnpConvention.cs

@@ -0,0 +1,43 @@
+//
+// Copyright (c) .NET Foundation and Contributors
+// Portions Copyright (c) Microsoft Corporation.  All rights reserved.
+// See LICENSE file in the project root for full license information.
+//
+
+using System;
+
+namespace nanoFramework.Azure.Devices.Provisioning.Client.PlugAndPlay
+{
+    /// <summary>
+    /// A helper class for formatting the DPS device registration payload, per plug and play convention.
+    /// </summary>
+    public static class PnpConvention
+    {
+        /// <summary>
+        /// Create the DPS payload to provision a device as plug and play.
+        /// </summary>
+        /// <remarks>
+        /// For more information on device provisioning service and plug and play compatibility,
+        /// and PnP device certification, see <see href="https://docs.microsoft.com/azure/iot-pnp/howto-certify-device"/>.
+        /// The DPS payload should be in the format:
+        /// <c>
+        /// {
+        ///   "modelId": "dtmi:com:example:modelName;1"
+        /// }
+        /// </c>
+        /// For information on DTDL, see <see href="https://github.com/Azure/opendigitaltwins-dtdl/blob/master/DTDL/v2/dtdlv2.md"/>
+        /// </remarks>
+        /// <param name="modelId">The Id of the model the device adheres to for properties, telemetry, and commands.</param>
+        /// <returns>The DPS payload to provision a device as plug and play.</returns>
+        /// <exception cref="ArgumentNullException">If modelId is <see langword="null"/> or <see cref="string.Empty"/>.</exception>
+        public static string CreateDpsPayload(string modelId)
+        {
+            if (string.IsNullOrEmpty(modelId))
+            {
+                throw new ArgumentNullException();
+            }
+
+            return $"{{\"modelId\":\"{modelId}\"}}";
+        }
+    }
+}