docs: add plugin setup guide

plugin/lib/commonlibf4/include/RE/F/FILE_HEADER.h

@@ -0,0 +1,14 @@
+#pragma once
+
+namespace RE
+{
+	class FILE_HEADER
+	{
+	public:
+		// members
+		float         version;     // 0
+		std::uint32_t formCount;   // 4
+		std::uint32_t nextFormID;  // 8
+	};
+	static_assert(sizeof(FILE_HEADER) == 0xC);
+}