[add] Docs about Database

This Docs is about "how database works".
At the moment it's a simple UML (I need to complete ASAP) don't take it
as default schema even because it's not integrated in the config.php
file.
Merge pull request #4 from ludo237/master

Pull Docs request to the master
Merge pull request #5 from PTKDev/master

merge
Merge pull request #6 from whilex/master

merge