kijiproject.github.com: DOCS-160. Add documentation for JpmmlScoreFunction.

issue: https://jira.kiji.org/browse/DOCS-160
review: https://review.kiji.org/r/1754
commit: https://github.com/kijiproject/kijiproject.github.com/commit/6c4bad9ff155f201a6b0695d30819717d671f71e