[docs] Update RELEASE.md for generating container tests for the slsa-verifier CLI

**Describe the bug**
Add a description and link to it in the RELEASE.md for how to generate verifier CLI tests when publishing new builders that release the container workflow.

See https://github.com/slsa-framework/slsa-github-generator/pull/1108

See issue on creating workflows: https://github.com/slsa-framework/example-package/issues/115