KIJIMRLIB-17. NaiveRecommendationProducer is incompatible with KijiTableKeyValueStoreReader.

Improved documentation to encourage users to appropriately override
NaiveRecommendationProducer#getRelatedItems(..).

issue: https://jira.kiji.org/browse/KIJIMRLIB-17
review: https://review.kiji.org/r/557/
reviewers: juliet