Added custom filter documentation.
Added custom filter documentation.

ongr-io/FilterManagerBundle,asev/FilterManagerBundle,saimaz/FilterManagerBundle,mansimas/FilterManagerBundle,mansimas/FilterManagerBundle,Dariusp/FilterManagerBundle,saimaz/FilterManagerBundle,murnieza/FilterManagerBundle,martiis/FilterManagerBundle,asev/FilterManagerBundle,murnieza/FilterManagerBundle,martiis/FilterManagerBundle,Dariusp/FilterManagerBundle