Update README.md

public class ReferenceServiceFactory extends ServiceFactory {

    public ReferenceServiceFactory() {

        registerService(StudentService.class, new ReferenceStudentService());

        // TODO: register other service implementations here

        registerService(CourseService.class, new ReferenceCourseService());

        // registerService(<interface name>.class, new <your implementation>());

    }

}

```

** After you have implemented your factory class, be sure to put your factory class name into the file `./config.properties`. So that we can find your implementation and test.**

```

serviceFactory=ReferenceServiceFactory            // Your factory class name here.

jdbcUrl=jdbc:postgresql://localhost:5432/project2

username=postgres

password=postgres

```

### Additional requirements of interface

#### Java