Add reference to slides for RMSprop (#5120)
Add reference to slides for RMSprop (#5120)

* Add reference to slides for RMSprop

I thought as the docs cite other methods, it would be good to provide a citation for this optimiser. Hinton mentions to 'keep citing the slides' for this method. 

I chose the title of the lecture in question, if there's a better title I'm sure that can be used instead, but I think the citation should be there.

* Whitespace violation

keras-team/keras,keras-team/keras