Update installation.rst

Base mapping file Order.orm.xml has a mapped-superclass. Fields cannot be override this way and gives a message like:  Duplicate definition of column 'id' on entity <your entity> in a field or discriminator column mapping. Removing all content inside the entity tags means it will take the defaults, exactly what you want. When you want to override it: Read the doctrine documentation: http://doctrine-orm.readthedocs.org/en/latest/reference/inheritance-mapping.html#attribute-override