Simple usage information

src/main.rs

@@ -15,6 +15,7 @@ fn main() {
             points[i][1] = values[1];
         } else {
             println!("Invalid input format: {}", part);
+            println!("Usage: $0 -1:1 3:-5 10:1.5")
         }
     }
     group_by_x(&mut points);