如何在Sencha Touch中向模型添加自定义验证规则

This article by Sencha covers how to use the built in validation rules (presence, length, format, inclusion, exclusion) and mentions that adding custom rules is easy, but doesn't ever explain how to do it. I've googled high and low and read the sencha docs, but I can't find anything on how to do it. Any Ideas?This article by Sencha covers how to use the bu