I realised now that this has not been included in the release notes, but it's probably the most important bit of the Csound5 distribution for the PD community:
Csound 5 includes csoundapi~, a PD frontend for csound in the style of the Max/MSP csound~ (which is based on csound4). Csoundapi~, unlike csound~ for PD, uses the csound library and API. It is multi-instantiable and provides many facilities for controlling csound. You will have access to the 1000 or so opcodes that csound provides (and that wonderful orchestra and score language that we all love :) ...).
Victor Lazzarini Music Technology Laboratory Music Department National University of Ireland, Maynooth