Link from basic middleware to middleware extensibility (#18367)

Author: serpent5

aspnetcore/fundamentals/middleware/write.md

@@ -5,7 +5,7 @@ description: Learn how to write custom ASP.NET Core middleware.
 monikerRange: '>= aspnetcore-2.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 5/6/2020
+ms.date: 5/18/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: fundamentals/middleware/write
 ---
@@ -15,6 +15,9 @@ By [Rick Anderson](https://twitter.com/RickAndMSFT) and [Steve Smith](https://ar
 
 Middleware is software that's assembled into an app pipeline to handle requests and responses. ASP.NET Core provides a rich set of built-in middleware components, but in some scenarios you might want to write a custom middleware.
 
+> [!NOTE]
+> This topic describes how to write *convention-based* middleware. For an approach that uses strong typing and per-request activation, see <xref:fundamentals/middleware/extensibility>.
+
 ## Middleware class
 
 Middleware is generally encapsulated in a class and exposed with an extension method. Consider the following middleware, which sets the culture for the current request from a query string: