rename `README.stub.template` using an `on: fork` workflow trigger

[dmarx]

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#fork

[dmarx]

maybe a simpler solution could be to add a mv script that runs if there are markdown files present other than README.md? otw I don't think I can cause modifications to the downstream fork (which makes a lot of sense and is a good thing security-wise)