add a note about having at least a dummy rule for rules[]

Closes: https://codeberg.org/dwl/dwl/issues/656

config.def.h

@@ -20,6 +20,7 @@ static const float fullscreen_bg[]         = {0.1f, 0.1f, 0.1f, 1.0f}; /* You ca
 /* logging */
 static int log_level = WLR_ERROR;
 
+/* NOTE: ALWAYS keep a rule declared even if you don't use rules (e.g leave at least one example) */
 static const Rule rules[] = {
 	/* app_id             title       tags mask     isfloating   monitor */
 	/* examples: */