[unity] Added remark for PMA not being supported by SpineVisualElement. See #2634.

spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Documentation/README.md

@@ -7,3 +7,7 @@ The plugin adds a `SpineVisualElement` control element to be used with UI Toolki
 
 For usage of the `SpineVisualElement` control element, you can check out the included `UI Toolkit Example.unity` example scene and `SpineUIToolkit.uxml` file which can be installed via the `Samples` tab in the Unity Package Manager window.
 The new `SpineVisualElement` can be found in the Unity `UI Builder` window under `Library - Project - Custom Controls` `Spine - Unity - SpineVisualElement`. It can be added to your hierarchy and configured as normal UI Toolkit elements while providing Spine skeleton attributes as well.
+
+### Limitations
+
+Premultiply-alpha (PMA) atlas textures are not supported due to limitations of the UI Toolkit `VisualElement` API, not allowing custom shaders. Please export your atlas textures as straight alpha to avoid artifacts.