Update migration for SignalR (#12305)

* Update migration for SignalR

* update

* Apply suggestions from code review

Co-Authored-By: BrennanConroy <brecon@microsoft.com>

* Apply suggestions from code review

Co-Authored-By: BrennanConroy <brecon@microsoft.com>

* sub header

* Update aspnetcore/migration/22-to-30.md

Co-Authored-By: BrennanConroy <brecon@microsoft.com>

* Update 22-to-30.md

* update the ms.date metadata value

* SignalR migration patch (#12352)

aspnetcore/migration/22-to-30.md

@@ -4,7 +4,7 @@ author: tdykstra
 description: Learn how to migrate an ASP.NET Core 2.2 project to ASP.NET Core 3.0.
 ms.author: tdykstra
 ms.custom: mvc
-ms.date: 04/17/2019
+ms.date: 05/09/2019
 uid: migration/22-to-30
 ---
 # Migrate from ASP.NET Core 2.2 to 3.0
@@ -437,35 +437,52 @@ The most significant change from `WebHostBuilder` to `HostBuilder` is in [depend
 
 ## Update SignalR code
 
-If you call `AddJsonProtocol`, replace it with `AddNewtonsoftJsonProtocol`.
+`System.Text.Json` is now the default Hub Protocol used by both the client and server.
 
-* The following examples show server code before and after the change:
+In `Startup.ConfigureServices`, call `AddJsonProtocol` to set serializer options.
 
-  ```csharp
-  services.AddSignalR(...)
-          .AddJsonProtocol(...) // 2.2
-  ```
+**Server:**
 
-  ```csharp
-  services.AddSignalR(...)
-          .AddNewtonsoftJsonProtocol(...) // 3.0
-  ```
+```csharp
+services.AddSignalR(...)
+        .AddJsonProtocol(options =>
+        {
+            options.WriteIndented = false;
+        })
+```
 
-* The following examples show .NET client code before and after the change:
+**Client:**
 
-  ```csharp
-  connection = new HubConnectionBuilder()
-      .WithUrl(...)
-      .AddJsonProtocol(...) // 2.2
-      .Build()
-  ```
+```csharp
+new HubConnectionBuilder()
+    .WithUrl("/chatHub")
+    .AddJsonProtocol(options =>
+    {
+        options.WriteIndented = false;
+    })
+    .Build();
+```
 
-  ```csharp
-  connection = new HubConnectionBuilder()
-      .WithUrl(...)
-      .AddNewtonsoftJsonProtocol(...) // 3.0
-      .Build()
-  ```
+### Switch to Newtonsoft.Json
+
+If you're using features of `Newtonsoft.Json` that aren't supported in `System.Text.Json`, you can switch back to `Newtonsoft.Json`:
+
+1. Install the [Microsoft.AspNetCore.SignalR.Protocols.NewtonsoftJson](https://www.nuget.org/packages/Microsoft.AspNetCore.SignalR.Protocols.NewtonsoftJson) NuGet package.
+1. On the client, chain an `AddNewtonsoftJsonProtocol` method call to the `HubConnectionBuilder` instance:
+
+    ```csharp
+    new HubConnectionBuilder()
+        .WithUrl("/chatHub")
+        .AddNewtonsoftJsonProtocol(...)
+        .Build();
+    ```
+
+1. On the server, chain an `AddNewtonsoftJsonProtocol` method call to the `AddSignalR` method call in `Startup.ConfigureServices`:
+
+    ```csharp
+    services.AddSignalR()
+        .AddNewtonsoftJsonProtocol(...);
+    ```
 
 ## Opt in to runtime compilation