Add assets for WordPress.org deployment via GitHub Actions

[helen]

Is your enhancement related to a problem? Please describe.
We are seeing increased usage of our GitHub Actions for deploying plugins to WordPress.org, and with the GA for Actions slated for mid-November and this being a “template” repo, I think it makes sense to go ahead and add what’s necessary for GitHub Actions to this repo so we’re ready from the jump. See 10up/action-wordpress-plugin-deploy#11 as well.

Describe the solution you'd like
Add .github/workflows/tag-deploy.yml, .github/workflows/asset-readme-update.yml, .distignore, and .wordpress-org/README.md. The last one should contain the image types, sizes, and naming that WordPress.org looks for, which I think makes more sense than bundling images that might unwittingly be deployed and avoids the empty directory in Git issue.

Designs
n/a

Describe alternatives you've considered
n/a

Additional context
I will work on a PR here within the next couple weeks unless somebody else wants to claim it first.