Create README for Generate Allowlist Stage

The generate allowlist stage does not have a README. This stage also does not have an section in the pipeline project's README. At a minimum, a section for this job must be added to the project README, and the stage may need its own README. @james.petersen, do you think a this stage should have a separate README from the project's, or will the addition of this stage in the project README be enough to document this stage?

Acceptance Criteria

- [ ] Provide decision on creation of new README, or only addition of stage to project README

conditional: If stage README should be created

• Update project README with addition of generate allowlist stage and provide link to stage's readme
• Creation of generate allowlist stage README