Mention SignalR Core 3.0's auto reconnect support (#15703)

* Mention SignalR Core 3.0's auto reconnect support

* Nits

* Update aspnetcore/signalr/version-differences.md

Co-Authored-By: Scott Addie <10702007+scottaddie@users.noreply.github.com>

* Update version-differences.md

* Update version-differences.md

* minor edits

aspnetcore/signalr/version-differences.md

@@ -4,7 +4,7 @@ author: bradygaster
 description: Differences between SignalR and ASP.NET Core SignalR
 monikerRange: '>= aspnetcore-2.1'
 ms.author: bradyg
-ms.date: 11/12/2019
+ms.date: 11/18/2019
 no-loc: [SignalR]
 uid: signalr/version-differences
 ---
@@ -28,7 +28,34 @@ ASP.NET Core SignalR isn't compatible with clients or servers for ASP.NET Signal
 
 ### Automatic reconnects
 
-Automatic reconnects aren't supported in ASP.NET Core SignalR. If the client is disconnected, the user must explicitly start a new connection if they want to reconnect. In ASP.NET SignalR, SignalR attempts to reconnect to the server if the connection is dropped.
+::: moniker range=">= aspnetcore-3.0"
+
+In ASP.NET SignalR:
+
+* By default, SignalR attempts to reconnect to the server if the connection is dropped. 
+* Automatic reconnects are opt-in with both the [.NET client](xref:signalr/dotnet-client#automatically-reconnect) and the [JavaScript client](xref:signalr/javascript-client#automatically-reconnect):
+
+```csharp
+HubConnection connection = new HubConnectionBuilder()
+    .WithUrl(new Uri("http://127.0.0.1:5000/chatHub"))
+    .WithAutomaticReconnect()
+    .Build();
+```
+
+```javascript
+const connection = new signalR.HubConnectionBuilder()
+    .withUrl("/chatHub")
+    .withAutomaticReconnect()
+    .build();
+```
+
+::: moniker-end
+
+::: moniker range="< aspnetcore-3.0"
+
+Prior to ASP.NET Core 3.0, SignalR doesn't support automatic reconnects. If the client is disconnected, the user must explicitly start a new connection to reconnect. In ASP.NET SignalR, SignalR attempts to reconnect to the server if the connection is dropped.
+
+::: moniker-end
 
 ### Protocol support