Discussion about designing the UX for "feature required":
https://ux.stackexchange.com/questions/11527/how-to-tell-a-client-in-a-form-that-they-can-enter-their-email-or-their-tel-b?noredirect=1&lq=1 https://ux.stackexchange.com/questions/21399/convention-for-showing-that-one-of-two-web-form-inputs-are-required
