code: public EntityBuilder addLink(Link link) {
      	if(link == null) {
      		throw new IllegalArgumentException("link cannot be null.");
      	}
      	addStep("_addLink", new Object[] { link }, true);
          return this;
      }
docs: Add a link to the entity to be built. Links are optional according to the Siren specification, however
  an entity should always have a 'self' link to be considered HATEAOS compliant.
  @param link cannot be <code>null</code> or empty.
  @return <code>this</code> builder, never <code>null</code>.