Added reference for pinv derivative
Added reference for pinv derivative

wkentaro/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,niboshi/chainer,pfnet/chainer,hvy/chainer,wkentaro/chainer