add VisibleForReflection

So, I've been thinking about how one could warn a user that a constructor or method really only exists for reflective access. This is mostly an issue when dealing within the same package these days.

JPA for example requires a public/protected constructor. I personally am always going to use a protected one, but that still shouldn't be called outside of subclasses, or maybe tests.

So my thought is an annotation like `VisibleForReflection` would be nice.