kiji-checkin: BENTO-74. Add a REVIEWERS file for kiji-checkin.

issue: https://jira.kiji.org/browse/BENTO-74
review: https://review.kiji.org/r/1811
commit: https://github.com/kijiproject/kiji-checkin/commit/bd7a3e7fce7b69d7cee2dd3e0ae452b62a3ccb7d