kijiproject.github.com: DOCS-131. First editing pass over overview section of Express userguide.

issue: https://jira.kiji.org/browse/DOCS-131
review: https://review.kiji.org/r/1321
commit: https://github.com/kijiproject/kijiproject.github.com/commit/c4fbb0d18f7d8880d0c2cd3e3c081c7e45213d4d