More migration docs.

Author: EliteMasterEric

src/09-migration/09-02-0.5.0-to-0.6.0.md

@@ -30,3 +30,27 @@ video.play(videoPath);
 // You can also run video.load() in advance before playing the video.
 if (video.load(videoPath)) video.play();
 ```
+
+# Options Menu Changes
+
+The options menu received a minor refactor internally. The `pages` list was moved to its own class, which changes the code needed to access the "Preferences" menu (mainly done to add custom preferences).
+
+We would like to standardize the process of adding custom user preferences to mods in the future eventually, but in the meantime you can make the necessary tweaks:
+
+```haxe
+// BEFORE: Retrieve the value from the page Map.
+if (Std.isOfType(currentState, OptionsState)) {
+    var preferencesPage = currentState.pages.get("preferences");
+
+    // Create a new option.
+    prefs.createPrefItemCheckbox(...);
+}
+
+// AFTER: Retrieve the value from the page Map, which is now inside a Codex.
+if (Std.isOfType(currentState, OptionsState)) {
+    var preferencesPage = currentState.optionsCodex.pages.get("preferences");
+
+    // Create a new option.
+    prefs.createPrefItemCheckbox(...);
+}
+```