Add description about SITE_ROOT in

Review Request #556 — Created Sept. 17, 2008 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
It tooks me half a day to discover that really useful line in the, simply because I understood that file was for internal Django config, and that no-one wanted to modify it. I finally felt on Christian's blog explaining that setting and then read it in the file.

Such a setting should be accessible to people configuring their install of reviewboard.
documentation modification