Update README.md

** 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.

serviceFactory=your.package.YourServiceFactory            // Your factory class name here.

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

username=postgres

password=postgres