Made documentation in `sproxy.sql` consistent with code in
`Authorize`.

The corresponding change in `Authorize` is here:
https://github.com/zalora/sproxy/commit/8cdc47bc841070305c97c980711843439c7fe093
sql: removed redundant JOIN in authorization query
README: added an explanation about the data model
README: added privileges example
Merge pull request #56 from zalora/sql_tweaks

Sql tweaks
Add timeout to specs
Move sending of response one level up when feasible
Support HTTP/1.0 for backend app (close #52)