Minimal post processing example

[poikilotherm]

Here's a thought for a very minimal first post processing thing, that also might address #172. This would be neat to have as part of the proof of concept.

Let's write during deposit a file with the DOI, revision_id etc and have a Github Action step get these details to create an issue or PR with a hint "hey, please add...". Scripted in Github for now, quick and dirty, but does not involve fiddling with Github API in Python for now. It would suit perfectly into the idea of providing templates, something we kind of neglected for the PoC.