Add README files to 2D demos

2d/kinematic_character/README.md

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+# Kinematic Character
+
+Example of how to make a kinematic character controller in 2D using
+[`KinematicBody2D`](https://docs.godotengine.org/en/latest/classes/class_kinematicbody2d.html).
+The character moves around, is affected by moving platforms,
+can jump through one-way collision platforms, etc. 
+
+Language: GDScript
+
+Renderer: GLES 2
+
+## Screenshots
+
+![Screenshot](screenshots/kinematic.png)