Update CONTRIBUTING.md (Azure#10817)

CONTRIBUTING.md

@@ -239,6 +239,69 @@ If you are adding a new service directory, ensure that it is mapped to a friendl
 8. Copy existing generate.ps1 file from another service and update the `ResourceProvider` name that is applicable to your SDK. Resource provider refers to the relative path of your REST spec directory in Azure-Rest-Api-Specs repository
    During SDK generation, this path helps to locate the REST API spec from the `https://github.com/Azure/azure-rest-api-specs`
 
+# On-boarding New generated code library
+
+1. Make a copy of `/sdk/template/Azure.Template` in you appropriate service directory and rename projects to `Azure.Management.*` for management libraries or `Azure.*` (e.g.  `sdk/storage/Azure.Management.Storage` or `sdk/storage/Azure.Storage.Blobs`)
+2. Modify `autorest.md` to point to you Swagger file or central README.md file. E.g.
+
+``` yaml
+input-file:
+    - https://raw.githubusercontent.com/Azure/azure-rest-api-specs/master/specification/storage/resource-manager/Microsoft.Storage/stable/2019-06-01/blob.json
+    - https://raw.githubusercontent.com/Azure/azure-rest-api-specs/master/specification/storage/resource-manager/Microsoft.Storage/stable/2019-06-01/file.json
+    - https://raw.githubusercontent.com/Azure/azure-rest-api-specs/master/specification/storage/resource-manager/Microsoft.Storage/stable/2019-06-01/storage.json
+```
+
+``` yaml
+require: https://github.com/Azure/azure-rest-api-specs/blob/49fc16354df7211f8392c56884a3437138317d1f/specification/azsadmin/resource-manager/storage/readme.md
+```
+
+3. Run `dotnet msbuild /t:GenerateCode` in src directory of the project (e.g. `net\sdk\storage\Azure.Management.Storage\src`). This would run Autorest and generate the code. (NOTE: this step requires Node 13).
+4. For management plan libraries add `azure-arm: true` setting to `autorest.md` client constructors and options would be auto-generated. For data-plane libraries follow the next two steps.
+4. Add a `*ClientOptions` type that inherits from `ClientOptions` and has a service version enum:
+
+``` C#
+namespace Azure.Management.Storage
+{
+    public class StorageManagementClientOptions: ClientOptions
+    {
+        private const ServiceVersion Latest = ServiceVersion.V2019_06_01;
+        internal static StorageManagementClientOptions Default { get; } = new StorageManagementClientOptions();
+
+        public StorageManagementClientOptions(ServiceVersion serviceVersion = Latest)
+        {
+            VersionString = serviceVersion switch
+            {
+                ServiceVersion.V2019_06_01 => "2019-06-01",
+                _ => throw new ArgumentOutOfRangeException(nameof(serviceVersion))
+            };
+        }
+
+        internal string VersionString { get; }
+
+        public enum ServiceVersion
+        {
+#pragma warning disable CA1707
+            V2019_06_01 = 1
+#pragma warning restore CA1707
+        }
+    }
+}
+```
+5. Add public constructors to all the clients using a partial class.
+``` C#
+ public partial class FileSharesClient
+    {
+        public FileSharesClient(string subscriptionId, TokenCredential tokenCredential): this(subscriptionId, tokenCredential, StorageManagementClientOptions.Default)
+        {
+        }
+
+        public FileSharesClient(string subscriptionId, TokenCredential tokenCredential, StorageManagementClientOptions options):
+            this(new ClientDiagnostics(options), ManagementClientPipeline.Build(options, tokenCredential), subscriptionId, apiVersion: options.VersionString)
+        {
+        }
+    }
+```
+
 ### Code Review Process
 
 Before a pull request will be considered by the Azure SDK team, the following requirements must be met: