docs: adding subject and main

subjects/java/checkpoints/monthly-period/ExerciseRunner.java

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+public class ExerciseRunner {
+    public static void main(String[] args) {
+        MonthlyPeriod MonthlyPeriod = new MonthlyPeriod();
+
+        // Test case 1
+        String startDate1 = "2020-01-01";
+        String endDate1 = "2023-06-15";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate1, endDate1));
+
+        // Test case 2
+        String startDate2 = "2015-05-20";
+        String endDate2 = "2015-10-20";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate2, endDate2));
+
+        // Test case 3
+        String startDate3 = "2018-12-25";
+        String endDate3 = "2021-12-25";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate3, endDate3));
+    }
+}

subjects/java/checkpoints/monthly-period/README.md

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+## Monthly Period
+
+### Instructions
+
+Create a class `MonthlyPeriod` that provides a method to calculate the period between two given dates in terms of months and years. The dates will be provided in the format `yyyy-MM-dd`.
+
+In case of any error the method `calculatePeriod` should return `Error`
+
+> 💡 Going to the past is possible!
+
+### Expected Class
+
+```java
+import java.time.LocalDate;
+import java.time.Period;
+import java.time.format.DateTimeFormatter;
+
+public class MonthlyPeriod {
+    public String calculatePeriod(String startDate, String endDate) {
+        // Implementation to calculate the period between two dates in months and years
+    }
+}
+```
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        MonthlyPeriod MonthlyPeriod = new MonthlyPeriod();
+
+        // Test case 1
+        String startDate1 = "2020-01-01";
+        String endDate1 = "2023-06-15";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate1, endDate1));
+
+        // Test case 2
+        String startDate2 = "2015-05-20";
+        String endDate2 = "2015-10-20";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate2, endDate2));
+
+        // Test case 3
+        String startDate3 = "2018-12-25";
+        String endDate3 = "2021-12-25";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate3, endDate3));
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+Period: 3 years and 5 months
+Period: 5 months
+Period: 3 years
+$
+```