Add "ProcessingPass" type of documentation
As we discussed briefly on Mattermost, we need to document the productions we make in the same way as the TWiki https://twiki.cern.ch/twiki/bin/view/Main/ProcessingPasses has been doing it for legacy data. In DPA we do not use TWiki pages and it seems natural to have production information together with the requests themselves anyway, hence in this repository.
We can see if something fancier will be necessary in the near future but for now I would suggest that we create a .md file in the top level, say PRODUCTIONS_INFO.md or alike, and document in there asap the relevant info on the handshake exercise. And we link the file from the README, very visibly :-).