clarify that write access to config.inc.php is required

access to `inc/poche/config.inc.php` is required in the case of MySQL installs. this commit clarifies that and also suggests a way to do that in Debian.
`db` folder is used for sqlite ;-)
And `inc/poche/config.php` is also created in SQLite installation. So your PR is available also for SQLite. 

I moved yesterday all user documentation into a new directory, can you upgrade your PR please? (I'm sorry ...)