code: public EntityBuilder addSubEntity(Entity subEntity) {
      	if(subEntity == null) {
      		throw new IllegalArgumentException("subEntity cannot be null.");
      	}
          addStep("_addEntity", new Object[] { subEntity }, true);
          return this;
      }
docs: Add a sub entity to the entity to be built.
  Sub entities are optional according to the Siren specification.
  @param subEntity cannot be <code>null</code>.
  @return <code>this</code> builder, never <code>null</code>.