📝 Developer note in boards.h

Marlin/src/core/boards.h

@@ -21,6 +21,11 @@
  */
 #pragma once
 
+/**
+ * Whenever changes are made to this file, please update Marlin/Makefile
+ * and _data/boards.yml in the MarlinDocumentation repo.
+ */
+
 #include "macros.h"
 
 #define BOARD_UNKNOWN -1