code: public EntityBuilder addAction(Action action) {
      	if(action == null) {
      		throw new IllegalArgumentException("action cannot be null.");
      	}
      	addStep("_addAction", new Object[] { action }, true);
          return this;
      }
docs: Add an action to the entity to be built. Actions are options according to the Siren specification.
  @param action cannot be <code>null</code>.
  @return <code>this</code> builder, never <code>null</code>.