Add mnist dataset directory to gitignore
Add mnist dataset directory to gitignore

chainer/chainer,wkentaro/chainer,jnishi/chainer,okuta/chainer,chainer/chainer,keisuke-umezawa/chainer,hvy/chainer,wkentaro/chainer,hvy/chainer,ktnyt/chainer,jnishi/chainer,chainer/chainer,jnishi/chainer,chainer/chainer,ktnyt/chainer,ktnyt/chainer,ktnyt/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,keisuke-umezawa/chainer,wkentaro/chainer,okuta/chainer,niboshi/chainer,keisuke-umezawa/chainer,pfnet/chainer,jnishi/chainer,hvy/chainer,tkerola/chainer,niboshi/chainer,okuta/chainer,niboshi/chainer