Add section on Flixel UI

src/09-migration/09-01-0.1.0-to-0.5.0.md

@@ -38,6 +38,14 @@ In your mod's `_merge` folder, look for any `json` files and rewrite their conte
 
 More information about this new system can be found at [Merging Files](10-appending-and-merging-files/10-02-merging-files.md).
 
+## Removal of Flixel UI
+
+[Flixel UI](https://github.com/haxeflixel/flixel-ui) is a library used for developing creating UI elements and managing UI events in HaxeFlixel. In the past, this was used to power the UI of the Chart Editor, but the development team regularly found the library to be frustrating to use, and eventually switched to [HaxeUI](https://github.com/haxeui) for most of its user interfaces. 
+
+In Friday Night Funkin' v0.5.0, the last places that the game used this library were refactored, and the game now exclusively uses a combination of manual sprite placement and HaxeUI for its user interfaces. As a result, Flixel UI was removed as a dependency.
+
+Any mods which utilized functions and classes provided by Flixel UI may need refactoring to compensate.
+
 ## Updating the API version
 
 Once all the migration steps above have been performed, the last step is to modify your mod's API version string. In your mod's `_polymod_meta.json` file, locate the `"api_version"` property and set it to `"0.5.0"`.