Include instructions on post-publish smoke tests

[danielbachhuber]

It would be helpful to have some smoke test instructions here: https://wordpress.github.io/wordpress-playground/docs/contributing/publishing#npm-packages

[adamziel]

• Visit https://translate.wordpress.org/projects/wp-plugins/contact-form-7/dev/de/default/playground/ since it uses the latest unpkg release
• Install the latest packages and confirm they can be imported – would be nice to have some script to run, or an automated check once that's a CI job