Add `gaussian_eta` to Parameters.md (#708)
Add `gaussian_eta` to Parameters.md (#708)

https://github.com/Microsoft/LightGBM/issues/705#issuecomment-315976423
Allardvm/LightGBM,microsoft/LightGBM,huanzhang12/LightGBM,microsoft/LightGBM,huanzhang12/LightGBM,henry0312/LightGBM,huanzhang12/LightGBM,Laurae2/LightGBM,olofer/LightGBM,huanzhang12/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,henry0312/LightGBM,Laurae2/LightGBM,Laurae2/LightGBM,microsoft/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,olofer/LightGBM,wxchan/LightGBM,olofer/LightGBM,Allardvm/LightGBM,olofer/LightGBM,huanzhang12/LightGBM,henry0312/LightGBM,wxchan/LightGBM,wxchan/LightGBM,microsoft/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,microsoft/LightGBM,olofer/LightGBM,henry0312/LightGBM,Laurae2/LightGBM,wxchan/LightGBM