Fix contribution guide
Fix contribution guide

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