add docs for norm_constraint
add docs for norm_constraint

DataFighter/Lasagne-tutorial,takacsg84/Lasagne,MihailoIsakov/fishfish,gabortakacs/Lasagne,ebattenberg/Lasagne,ErnstHowie/Lasagne,dnuffer/Lasagne,kracwarlock/Lasagne,mheilman/Lasagne,DeanChan/Lasagne,Richi91/nntools,cancan101/Lasagne,rlkelly/Lasagne,diogo149/Lasagne