Add a script and accompanying one-liner in docs for basic workflow setup

[helen]

Is your enhancement related to a problem? Please describe.
There's still a little bit of a learning curve when it comes to getting set up with Actions. We have instructions but they aren't quite at the level of "make a file in X directory and paste this in". New plugins could potentially use 10up/plugin-scaffold#104 once that's merged, but that doesn't help existing plugins.

Describe the solution you'd like
I'd like a small shell script that can be curl/wget-ed directly from GH that writes the basic files you need into place. I don't think it should bother with input and rather just show you a message after completion prompting you with things to do next (set a SLUG, add and commit), but can be convinced otherwise. The script should probably delete itself when done.

Designs
n/a

Describe alternatives you've considered
Adding more step-by-step docs, which we should probably also do.

Additional context
This is inspired by the release of https://github.com/ahmadawais/wp-continuous-deployment (which is also a perfectly good tool in its own right)