includes/signalr-updated-version Create include to drive traffic from…

… SignalR to SignalR Core dotnet#5700 (dotnet#5761)

* Create include showing signalr core version docs location

* Update signalr-updated-version.md

aspnet/includes/signalr-updated-version.md

@@ -0,0 +1 @@
+`The preview version of SignalR for ASP.NET Core documentation is [here](https://docs.microsoft.com/aspnet/core/signalr/introduction).`