Document Program.withNotFound

src/Website/docs/Routing.md

@@ -70,6 +70,15 @@ Here are the steps to set up an inferred router:
     |> Program.withRouter router
     ```
 
+You can specify the endpoint to show when the user visits a URL that can't be parsed with `Program.withNotFound`.
+Note that this endpoint is used when initially loading a page with an unrecognized URL; however it is not used when the user clicks a link in the Bolero page to an unrecognized URL, as that would prevent using external links.
+
+```fsharp
+Program.mkSimple initModel update view
+|> Program.withRouter router
+|> Program.withNotFound Home
+```
+
 > Note: the message `PageChanged` is dispatched automatically when the URL has been modified (eg by clicking a link).
 > It is not recommended to dispatch it directly yourself, as it will result in the same message being dispatched twice: first by you, and then by the router.
 >