docs: add plugin setup guide

plugin/lib/commonlibf4/include/RE/G/GROUP_REACTION.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "RE/F/FIGHT_REACTION.h"
+
+namespace RE
+{
+	class GROUP_REACTION
+	{
+	public:
+		// members
+		TESForm*       form;           // 00
+		std::int32_t   reaction;       // 08
+		FIGHT_REACTION fightReaction;  // 0C
+	};
+	static_assert(sizeof(GROUP_REACTION) == 0x10);
+}