docs: adding subject

subjects/java/checkpoints/helloworld/README.md

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+## HelloWorld
+
+### Instructions
+
+In a file named `HelloWorld.java` write a function `helloWorld` that returns the string 'Hello World !'.
+
+### Expected Functions
+
+```java
+public class HelloWorld {
+    public static String helloWorld() {
+        // your code here
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your function :
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        System.out.println(HelloWorld.helloWorld());
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Hello World !
+$
+```

subjects/java/checkpoints/system-logger/README.md

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+## SystemLogger
+
+### Instructions
+
+In a file named `SystemLog.java` write a function `systemLog` that takes a String message as a parameter and returns the given message concatenated with 'System Log: '
+
+### Expected Functions
+
+```java
+public class SystemLog {
+    public static String systemLog(String message) {
+        // your code here
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your function :
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        System.out.println(SystemLog.systemLog('message'));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+System Log: message
+$
+```

subjects/java/checkpoints/universal-greeting/README.md

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+## UniversalGreeting
+
+### Instructions
+
+In a file named `UniversalGreeting.java` write a function `greeting` that take a String language as parametre and return a greating message based on the language as follows:
+
+FR : Bonjour comment allez-vous
+EN : Hello, How are you
+ES : Hola, cómo estás
+
+
+### Expected Functions
+
+```java
+public class UniversalGreeting {
+    public static String greeting(String message) {
+        // your code here
+    }
+}
+```
+
+### Usage
+
+Here is a possible ExerciseRunner.java to test your function :
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        System.out.println(UniversalGreeting.greeting('EN'));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Hello, How are you
+$
+```