Update documentation - promises and goals

[PurelyApplied]

The following need to be included or updated in our documentation.

• Explicitly document what taint propagation we do and do not attempt to track.
• Document existing inference, i.e., given a source type Foo, what other types do we implicitly consider sources?
• State that we do not handle reflection explicitly, though some cases may be handled by general taint propagation.

[mlevesquedion]

From a comment by @PurelyApplied on #292:

We should consider explicitly drawing attention to "you should make sure you trust your errors" when we update documentation surrounding the scope of taint propagation.