Explain how to compute effective failure level based on configuration

One contributor to a result's default display state in a viewer (see #23) is its level. The spec explains how the effect level is computed in a complicated way based on result.level, rule.defaultConfiguration.level, and applied policy.

Add an Appendix that describes this algorithm (which viewers must implement, and for which there really should be an SDK API) in terms a human can understand.

@michaelcfanning FYI.