Added reference to inverted dropout
Added reference to inverted dropout

mitchellstern/dynet,kamigaito/cnn,Peilun-Li/dynet,cherryc/dynet,Abasyoni/dynet,Abasyoni/dynet,Peilun-Li/dynet,Abasyoni/dynet,Peilun-Li/dynet,mitchellstern/dynet,cherryc/dynet,cherryc/dynet,kamigaito/cnn