Backport documentation on integrating Symfony2 with the PropelBundle

As far as I can understand, Propel2 will only allow [Symfony 2.3+](https://github.com/propelorm/Propel2/blob/da32413a8ceb295d3bb708dc311756c724a21c7b/composer.json#L20-L25) so this is certainly not needed, what do you think ? Moreover, if Symfony < 2.3 is allowed, should we really document its features in our own documentation ?