Add documentation for poetry-black.yml workflow

[koeaw]

• adds an .md file which documents how to use the existing workflow to the docs dir
• updates the README with a link to the new file

[koeaw]

The only thing I'm not entirely sure about is the path filter – is this a bad recommendation for our use case? Will it make Black ignore other files which might be relevant?!

[koeaw]

I would remove the whole Workflow triggers paragraph, as it duplicates upstream documentation and is not specific to this wofklow.

Yeah, idk anymore why I included this. Maybe I was thinking of other examples or active workflows where more keywords are being used (?) and wanted to add this as warning of sorts in case someone copy-pastes settings unaware of possible side effects.

But the example here only uses pull_request and if we assume someone wanting to extend the workflow would look at the docs, we can also assume they'll likely come across info about this potential pitfall.

[koeaw]

Removed the section on workflow triggers -> merging.