add demo for reduce

src/main/java/feature/ReduceTest.java

@@ -0,0 +1,14 @@
+package feature;
+
+import java.util.List;
+
+public class ReduceTest {
+
+    public static void main(String[] args) {
+        List<Integer> numbers = List.of(1, 2, 3, 4, 5);
+        int sum = numbers.stream()
+                .reduce(0, Integer::sum);
+
+        System.out.println(sum);
+    }
+}