Added some comment in day 5 code

src/day5.rs

@@ -42,6 +42,7 @@ impl Day5 {
         let mut prev = HashSet::new();
         for n in update {
             if let Some(rule) = self.rule_map.get(n) {
+                // The rule_map maps from a number A to a set of numbers that should come after it. So if one of those numbers was seen previously, this rule is in the wrong order.
                 if rule.iter().any(|n2| prev.contains(n2)) {
                     return false;
                 }