Add steps to example workflow around ZIP generation

[helen]

Is your enhancement related to a problem? Please describe.
With the new .distignore method of excluding files from deployment, the GitHub-generated ZIP file attached to releases no longer reflects what gets deployed to .org. Also, broadly that automatic naming is an issue, as you'll get plugin-slug-X.Y.Z as the folder name upon unzipping, which is not really what you or WordPress want.

Describe the solution you'd like
Test additional steps in a test repo workflow and then add them to README.md. My sense is that existing Actions can be leveraged to generate the ZIP and attach it to a release, but don't know yet. If not, we can write one, which will also be useful for plugins that don't release to .org at all and update directly from GH instead.

[shivapoudel]

Ref: 10up/actions-wordpress#8 (comment)