feat: add introduction with observable and side effects operator

src/main/java/com/ippon/codingdojo/reactive/part2/Code6Solution.java

+package com.ippon.codingdojo.reactive.part2;
+
+import io.reactivex.Observable;
+
+public class Code6Solution {
+
+  public static void main(String[] args) {
+
+    final Observable<String> observable = Observable.create(subscriber -> {
+      subscriber.onNext("Alice");
+      subscriber.onNext("John");
+      subscriber.onError(new RuntimeException("BAZINGA"));
+      subscriber.onComplete();
+    });
+
+    observable
+        .subscribe(
+            System.out::println,
+            e -> System.out.println("Ouuups : " + e.getMessage()),
+            () -> System.out.println("onComplete()")
+        );
+
+  }
+
+}