add explanation of perplexity
add explanation of perplexity

ktnyt/chainer,pfnet/chainer,chainer/chainer,okuta/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,keisuke-umezawa/chainer,jnishi/chainer,ktnyt/chainer,jnishi/chainer,tkerola/chainer,jnishi/chainer,rezoo/chainer,chainer/chainer,wkentaro/chainer,keisuke-umezawa/chainer,hvy/chainer,okuta/chainer,aonotas/chainer,niboshi/chainer,wkentaro/chainer,niboshi/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,ronekko/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,ktnyt/chainer,chainer/chainer,jnishi/chainer,anaruse/chainer,niboshi/chainer,ktnyt/chainer