docs: adding subject

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

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+## SortArray
+
+### Instructions
+
+In a file named `SortArray.java` write a function `sort` that sorts ascending the given array specified in the parameters and returns it.
+
+### Expected Functions
+
+```java
+public class SortArray {
+    public static Integer[] sort(Integer[] args) {
+        // 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(SortArray.sort(new Integer[]{4, 2, 1, 3})));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+[1, 2, 3, 4]
+$
+```