Updating usage instructions in README

What does this MR do?

As noted in the related issue, this analyzer requires two files to exist in customer repositories. This MR updates the README to add this nuance to the project's README.

What are the relevant issue numbers?

gitlab-org/gitlab#199189 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer