docs: add plugin setup guide

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

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "Scaleform/G/GFx_State.h"
+
+namespace Scaleform::GFx
+{
+	class Movie;
+	class Value;
+
+	class __declspec(novtable) ExternalInterface :
+		public State  // 00
+	{
+	public:
+		ExternalInterface() :
+			State(StateType::kExternalInterface)
+		{}
+
+		virtual ~ExternalInterface() = default;  // 00
+
+		// add
+		virtual void Callback(Movie* a_movieView, const char* a_methodName, const Value* a_args, std::uint32_t a_numArgs) = 0;  // 01
+	};
+	static_assert(sizeof(ExternalInterface) == 0x18);
+}