Add README files to loading and multiplayer demos

loading/autoload/README.md

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+# Autoload Demo
+
+This demo shows how to use autoloads to change between scenes.
+
+Language: GDScript
+
+Renderer: GLES 2

loading/background_load/README.md

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+# Background Load
+
+This is a demo showing how to use the `load_interactive()` method of
+[`ResourceLoader`](https://docs.godotengine.org/en/latest/classes/class_resourceloader.html)
+to load large scenes in the background with a progress bar.
+
+Language: GDScript
+
+Renderer: GLES 2
+
+## Screenshots
+
+![Screenshot](screenshots/paintings.png)

loading/scene_changer/README.md

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+# Scene Changer
+
+This uses functions in SceneTree to switch between two scenes.
+
+Language: GDScript
+
+Renderer: GLES 2

loading/threads/README.md

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+# Loading with Threads
+
+An example using a thread to load an image. 
+
+Language: GDScript
+
+Renderer: GLES 2
+
+## Screenshots
+
+![Screenshot](screenshots/load_in_thread.png)