kiji-express: EXP-253.  Update README.md for new KijiInput/Output APIs.

review: https://review.kiji.org/r/1273
issue: https://jira.kiji.org/EXP-253
commit: https://github.com/kijiproject/kiji-express/commit/98d70718affa9d336a37c952b23206b01a678938