Require a main function in all exercises

exercises/11_hashmaps/hashmaps3.rs

@@ -5,9 +5,9 @@
 // Example: England,France,4,2 (England scored 4 goals, France 2).
 //
 // You have to build a scores table containing the name of the team, the total
-// number of goals the team scored, and the total number of goals the team 
-// conceded. One approach to build the scores table is to use a Hashmap. 
-// The solution is partially written to use a Hashmap, 
+// number of goals the team scored, and the total number of goals the team
+// conceded. One approach to build the scores table is to use a Hashmap.
+// The solution is partially written to use a Hashmap,
 // complete it to pass the test.
 //
 // Make me pass the tests!
@@ -42,6 +42,10 @@ fn build_scores_table(results: String) -> HashMap<String, Team> {
     scores
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;