docs: add plugin setup guide

plugin/lib/commonlibf4/include/Scaleform/G/GFx_FSCommandHandler.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "Scaleform/G/GFx_State.h"
+
+namespace Scaleform::GFx
+{
+	class Event;
+	class Movie;
+
+	class __declspec(novtable) FSCommandHandler :
+		public State  // 00
+	{
+	public:
+		static constexpr auto GFX_STATE_TYPE{ StateType::kUserEventHandler };
+
+		FSCommandHandler() :
+			State(StateType::kFSCommandHandler)
+		{}
+
+		virtual void HandleEvent(class Movie* pmovie, const class Event& event) = 0;
+	};
+	static_assert(sizeof(FSCommandHandler) == 0x18);
+}