docs: adding subject and main

subjects/java/checkpoints/singleton-blueprint/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        System.out.println(GoodbyeMars.goodbyeMars());
+    }
+}

subjects/java/checkpoints/singleton-blueprint/README.md

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+## Singleton Blueprint
+
+### Instructions
+
+You are given an incomplete singleton class. Complete the class to demonstrate your understanding of how the Singleton design pattern works. The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance.
+
+### Expected Class
+
+```java
+public class Singleton {
+    public static Singleton instance;
+
+    private Singleton() {
+        // Initialization code
+    }
+
+    public static Singleton get???() {
+        // Implementation to return the single instance
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        // Get the only object available
+        Singleton singleton = Singleton.getInstance();
+
+        // Show the message
+        singleton.showMessage();
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Hello, I am a singleton!
+$
+```