feat: add elements

game-design/gameplay/battlesystem/elements.md

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+## Elements
+
+Like Sonic Chronicles, Radiance will use a slight Element system, in order to add strenths and weaknesses to ennemies and characters.
+
+The six elements are :
+- Water : Aquatic badnics
+- Fire : Fire-using badnics
+- Ice : Snow level badnics
+- Wind : Aerial badnics
+- Earth : underground badnics
+- Light(ning) : Electricity-using badnics.
+
+A seventh is the chaos, that is weak to itself. It's the element of Phi-based bots, Android Shadows, etc.
+
+Each elements is weak to the previous on the list (Water -> Fire -> Ice -> Wind -> Earth -> Light), and resists to itself and to the next.
+
+Each heroes will have one elemental weakness and one resistence.
+
+Each badnics will have an element, and will follow the normal circle.