Update README.md

Author: janbodnar

fun-streams/reduce/README.md

@@ -1 +1,23 @@
 # The reduce operation
+
+
+
+
+
+
+## reduce example
+
+```java
+import java.util.List;
+import java.util.Optional;
+
+void main() {
+
+    List<Integer> nums = List.of(1, 2, 3, 4, 5);
+    Optional<Integer> res = nums.stream().reduce((a, b) -> a + b);
+    res.ifPresent(System.out::println);    
+
+    Optional<Integer> res3 = nums.stream().reduce(Integer::sum);
+    res3.ifPresent(System.out::println);  
+}
+```