Document database schema

Summary:
Please review carefully, me not very well on English.

I am just guessing what were the design decisions in most parts of this document.
Feel free to correct me or add more information.

Test Plan:
diviner .
/docs/
/docs/article/Database_Schema.html
Copy the text to Word and proofread.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: nh, jungejason, aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2258