Update browser.md (#17636)

Author: JamesNK

aspnetcore/grpc/browser.md

@@ -43,6 +43,11 @@ Alternatively, configure all services to support gRPC-Web by adding `services.Ad
 
 [!code-csharp[](~/grpc/browser/sample/AllServicesSupportExample_Startup.cs?name=snippet_1&highlight=6,13)]
 
+> [!NOTE]
+> There is a known issue that causes gRPC-Web to fail when [hosted by Http.sys](xref:fundamentals/servers/httpsys) in .NET Core 3.x.
+>
+> A workaround to get gRPC-Web working on Http.sys is available [here](https://github.com/grpc/grpc-dotnet/issues/853#issuecomment-610078202).
+
 ### gRPC-Web and CORS
 
 Browser security prevents a web page from making requests to a different domain than the one that served the web page. This restriction applies to making gRPC-Web calls with browser apps. For example, a browser app served by `https://www.contoso.com` is blocked from calling gRPC-Web services hosted on `https://services.contoso.com`. Cross Origin Resource Sharing (CORS) can be used to relax this restriction.