FYI - Next Stan release (2.21) October 18 - now with feature freeze

Thanks for the notice – I still need to expose the newton solver. I can brute-force do it through stan, and then we can hopefully get it reviewed. That said, I want to this in stanc3, for which I’ve had some issues.

Also, when do we require all the documentation to be written? Does it have to be before the feature freeze?