docs: add plugin setup guide

plugin/lib/commonlibf4/include/RE/E/ExteriorCellSingleton.h

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+#pragma once
+
+#include "RE/B/BSTSingleton.h"
+#include "RE/N/NiPointer.h"
+
+namespace RE
+{
+	class bhkWorldM;
+
+	class ExteriorCellSingleton :
+		public BSTSingletonExplicit<ExteriorCellSingleton>
+	{
+	public:
+		[[nodiscard]] static ExteriorCellSingleton* GetSingleton()
+		{
+			static REL::Relocation<ExteriorCellSingleton**> singleton{ ID::ExteriorCellSingleton::Singleton };
+			return *singleton;
+		}
+
+		//members
+		NiPointer<bhkWorldM> exteriorWorld;
+	};
+	static_assert(sizeof(ExteriorCellSingleton) == 0x8);
+}