Add more explanation about the DAOs

[moufmouf]

In https://github.com/thecodingmachine/database.doctrine-orm-wrapper/blob/master/doc/daos.md , you are talking about the fact that having DAOs provides an "auto-completion" feature in the IDE. This is true, but this is not the only reason we are designing DAOs.

The main reason is that DAOs are designed to separate cleanly the code between that performs requests to the database from the rest of the world.

Therefore, any DQL request, any request at all actually should be scoped in the DAOs.

We might want to give a sample of what an actual DAO should look like (and explain in bold that the BaseDao must not be modified.