docs: adding subject (#2528)

subjects/java/checkpoints/rotate-array/README.md

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+## RotateArray
+
+### Instructions
+
+In a file named `RotateArray.java` write a function `rotate` that rotates the given array specified in the parameters by the number given in the second parameter and returns it.
+
+> Note: The rotation count may have a negative value!!!
+
+### Expected Functions
+
+```java
+public class RotateArray {
+    public static Integer[] rotate(Integer[] arr, int rotationCount) {
+        // your code here
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your function :
+
+```java
+import java.io.*;
+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) throws IOException {
+        System.out.println(Arrays.toString(RotateArray.rotate(new Integer[]{4, 2, 1, 3}, 1)));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+[3, 4, 2, 1]
+$
+```