README: Add a note about render loop type

Author: adazem009

README.md

@@ -114,6 +114,13 @@ Button {
 }
 ```
 
+Please note that the ScratchCPP renderer only works with the basic scene graph render loop.
+Qt 6 uses the threaded render loop by default, so you'll have to disable it by calling this
+before constructing your application object:
+```cpp
+qputenv("QSG_RENDER_LOOP", "basic");
+```
+
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