GitHub auto-merge instructions

[barneydobson]

I think for the auto-merge to work as intended, the created GitHub repo needs to "allow auto-merge" under PR in general settings, and "Allow GitHub Actions to create and approve pull requests" under settings->actions->general?

I don't know if these should be specified in the readme, as I even know to do this and forget half the time, so someone who doesn't know may be confused why they keep getting PRs from dependabot staying open.

Or possibly this can be accommodated by the cookiecutter template, not sure.