document blur validationTrigger and custom elements

http://stackoverflow.com/questions/39187287/aurelia-validation-not-binding-to-textbox-component/39193353#39193353

Start with the linked Stack Overflow question to understand the requested blur validationTrigger and custom-elements behavior. Then locate the repository's existing validation documentation and document the relevant configuration and usage clearly; done means a newcomer can find and follow the guidance.

Written by the indexing model from the issue text.