Fix contribution guide
Fix contribution guide

ktnyt/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer,AlpacaDB/chainer,niboshi/chainer,pfnet/chainer,kashif/chainer,okuta/chainer,kiyukuta/chainer,keisuke-umezawa/chainer,kikusu/chainer,anaruse/chainer,okuta/chainer,cupy/cupy,aonotas/chainer,okuta/chainer,ysekky/chainer,ktnyt/chainer,cupy/cupy,hvy/chainer,AlpacaDB/chainer,hvy/chainer,ronekko/chainer,delta2323/chainer,hvy/chainer,jnishi/chainer,chainer/chainer,keisuke-umezawa/chainer,tkerola/chainer,benob/chainer,ktnyt/chainer,jnishi/chainer,chainer/chainer,cupy/cupy,cupy/cupy,wkentaro/chainer,ktnyt/chainer,wkentaro/chainer,niboshi/chainer,rezoo/chainer,niboshi/chainer,chainer/chainer,keisuke-umezawa/chainer,kikusu/chainer,jnishi/chainer,wkentaro/chainer,chainer/chainer,benob/chainer,jnishi/chainer,okuta/chainer,niboshi/chainer