kijiproject.github.com: DOCS-140. Add new Express UG content.

issue: https://jira.kiji.org/browse/DOCS-140
review: https://review.kiji.org/r/1422
commit: https://github.com/kijiproject/kijiproject.github.com/commit/c0cefa5730469b094aa0ad8b29140a9097f747ce