Backport documentation on integrating Symfony2 with the PropelBundle

Nitpcking again but you should either link to the actual reference file (i.e. [Configuration Properties Reference](https://github.com/propelorm/propelorm.github.com/blob/4a389865314d4cb0f02cb2476bb144d3866585e4/documentation/reference/configuration-file.markdown)) or leave the link as is and say "for further details", what do you think ?