Add descriptions to all projects with README files

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2d/platformer/project.godot

@@ -51,6 +51,13 @@ _global_script_class_icons={
 [application]
 
 config/name="Platformer 2D"
+config/description="This demo is a pixel art 2D platformer with graphics and sound.
+
+It shows you how to code characters and physics-based objects
+in a real game context. This is a relatively complete demo
+where the player can jump, walk on slopes, fire bullets,
+interact with enemies, and more. It contains one closed
+level, and the player is invincible, unlike the enemies."
 run/main_scene="res://src/Main/Game.tscn"
 config/icon="res://icon.png"
 target_fps="60"