[Checkers] New material for checkers paper

[Michael Ernst]

Matt-

> - framework meta-annotations (manual section 8.5.1)
> - polymorphism (not done yet, will be in SVN in the next couple of
> days; can actually be part of the meta-annotations discussion, since
> it's done via a @Polymorphic meta-annotation)

I would put the description of this near the flow-sensitive discussion.  In
the discussion of meta-annotations, it can be mentioned that a
meta-annotation enables it.

> - nonnull @Raw types (manual section 3.3)
> - custom checker (manual section 8.1)

In addition to mentioning the existence of the custom checker, is there a
case study to add?

                    -Mike