docs: adding subject

subjects/java/checkpoints/do-operation/README.md

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+## DoOperation
+
+### Instructions
+
+In a file named `DoOperation.java` write a function `operate` that returns the result of the given arithmetic operation specified in the parameters. The arguments should be passed in the following order:
+
+- The first argument is the left operand
+- The second argument is the operation sign
+- The third argument is the right operand
+
+### Expected Functions
+
+```java
+public class DoOperation {
+    public static String operate(String[] args) {
+        // 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(DoOperation.operate(new String[]{"1","+","2"}));
+        System.out.println(DoOperation.operate(new String[]{"1","-","1"}));
+        System.out.println(DoOperation.operate(new String[]{"1","%","0"}));
+        System.out.println(DoOperation.operate(args));
+    }
+}
+```
+
+and its output :
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+3
+0
+Error
+it depend on the input.
+$
+```