[#445] Update code reuse (#446)

Update code reuse

src/test/java/seedu/address/testutil/PersonBuilder.java

@@ -17,6 +17,7 @@
  */
 public class PersonBuilder<T extends PersonBuilder<T>> {
 
+    // Solution inspired by https://stackoverflow.com/questions/17164375/subclassing-a-java-builder-class
     public static final String DEFAULT_NAME = "Amy Bee";
     public static final String DEFAULT_PHONE = "85355255";
     public static final String DEFAULT_EMAIL = "[email protected]";