Create Pong with GDNative C++ demo

cpp/pong/README.md

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+# Pong with GDNative C++
+
+A simple Pong game, made with GDNative C++. This demo shows best practices
+for game development in Godot, including
+[signals](https://docs.godotengine.org/en/latest/getting_started/step_by_step/signals.html).
+
+Language: C++
+
+Renderer: GLES 2
+
+Note: There is a GDScript version available [here](https://github.com/godotengine/godot-demo-projects/tree/master/2d/pong).
+
+Note: There is a C# version available [here](https://github.com/godotengine/godot-demo-projects/tree/master/mono/pong).
+
+Note: There is a VisualScript version available [here](https://github.com/godotengine/godot-demo-projects/tree/master/visual_script/pong).
+
+## Compiling
+
+You can use SCons to compile the library:
+
+```
+scons platform=PLATFORM
+```
+
+Where PLATFORM is: `windows`, `linux`, or `osx`.
+
+This creates the file `libsimple` in the respective
+subfolders in the `project/gdnative` directory.
+
+Dependencies:
+ * You need [godot-cpp](https://github.com/godotengine/godot-cpp),
+   this is now a Git submodule of this repo.
+ * `clang`, `gcc`, or any decent C compiler that's C++14 compatible.
+
+## How does it work?
+
+The walls, paddle, and ball are all
+[`Area2D`](https://docs.godotengine.org/en/latest/classes/class_area2d.html)
+nodes. When the ball touches the walls or the paddles,
+they emit signals and modify the ball.
+
+## Screenshots
+
+![Screenshot](screenshots/pong.png)