Added note for working with virtual directories (#9128)

I spent a day or more researching this so that I could use Swagger-UI in my ASP.NET Core API which is used with a reverse-proxy and virtual directory.  In the end found Swagger-UI requires relative URLs to start with a period.  Related to https://github.com/swagger-api/swagger-ui/issues/4107

Hoping this documentation note will help others.

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aspnetcore/tutorials/getting-started-with-swashbuckle.md

@@ -4,7 +4,7 @@ author: zuckerthoben
 description: Learn how to add Swashbuckle to your ASP.NET Core web API project to integrate the Swagger UI.
 ms.author: scaddie
 ms.custom: mvc
-ms.date: 11/05/2018
+ms.date: 11/14/2018
 uid: tutorials/get-started-with-swashbuckle
 ---
 # Get started with Swashbuckle and ASP.NET Core
@@ -102,6 +102,8 @@ The Swagger UI can be found at `http://localhost:<port>/swagger`. Explore the AP
 >
 > [!code-csharp[](../tutorials/web-api-help-pages-using-swagger/samples/2.0/TodoApi.Swashbuckle/Startup3.cs?name=snippet_UseSwaggerUI&highlight=4)]
 
+If using virtual directories (with IIS or a reverse proxy, for example), set the Swagger endpoint to a relative path using the `./` prefix. For example, `./swagger/v1/swagger.json`. Using `/swagger/v1/swagger.json` instructs the app to look for the JSON file at the true root of the URL (plus the route prefix, if used). For example, `http://localhost:<port>/<route_prefix>/swagger/v1/swagger.json` instead of `http://localhost:<port>/<virtual_directory>/<route_prefix>/swagger/v1/swagger.json`.
+
 ## Customize and extend
 
 Swagger provides options for documenting the object model and customizing the UI to match your theme.