kiji-model-repository: KIJIREPO-30. Write documentation around setup, installation and usage of the model repository

issue: https://jira.kiji.org/browse/KIJIREPO-30
review: https://review.kiji.org/r/1403
commit: https://github.com/kijiproject/kiji-model-repository/commit/83ae91c98b9c5d828d10588b5489465064e21799