Backport documentation on integrating Symfony2 with the PropelBundle

Maybe you can set `configuration: true` at the top of this document and rely on the syntax used [here](https://github.com/propelorm/propelorm.github.com/blob/4a389865314d4cb0f02cb2476bb144d3866585e4/documentation/07-logging.markdown#logger-configuration) for instance to showcase the different syntax according to the used markup, what do you think ?