docs: Explain how to log in.

Closes #970

templates/error/descriptions/H401.md.hbs

@@ -1,4 +1,16 @@
 # Not logged in
-This resource is only accessible by logged in agents.
-
+**This resource is only accessible by logged in agents.**
+<br>
 Try logging in and revisiting this resource.
+
+**To log in and access documents, you need to use a Solid app.**
+<br>
+This server provides secure storage,
+but it is not a client app.
+<br>
+Choose one of the [Solid apps](https://solidproject.org/apps)
+to log in and browse Pods.
+
+Alternatively,
+[configure](https://github.com/solid/community-server-recipes)
+this server with a default application.