Add sample code for avatar claim

Add some sample code to the documentation showing how to map the user's avatar URL to a claim.

docs/discord.md

@@ -20,3 +20,26 @@ _None._
 | Property Name | Property Type | Description | Default Value |
 |:--|:--|:--|:--|
 | `Prompt` | `string?` | The value to use for the `prompt` query string parameter when making HTTP requests to the authorization endpoint. | `null` |
+
+## Avatars as Claims
+
+Versions of the Discord provider before version `6.0.0` would automatically map the user's avatar URL as the `urn:discord:avatar:url` claim.
+
+This functionality is no longer built-in (see #584 and #585), but can be added to your application with some extra code similar to that shown in the sample below.
+
+```csharp
+services.AddAuthentication(options => /* Auth configuration */)
+        .AddDiscord(options =>
+        {
+            options.ClientId = "my-client-id";
+            options.ClientSecret = "my-client-secret";
+
+            options.ClaimActions.MapCustomJson("urn:discord:avatar:url", user =>
+                string.Format(
+                    CultureInfo.InvariantCulture,
+                    "https://cdn.discordapp.com/avatars/{0}/{1}.{2}",
+                    user.GetString("id"),
+                    user.GetString("avatar"),
+                    user.GetString("avatar").StartsWith("a_") ? "gif" : "png"));
+        });
+```