Explain automatic release process in readme

What does this MR do?

This MR updates the readme to explain the automatic analyzer release process as implemented in Automatically tag and release secure analyzers

What are the relevant issue numbers?

gitlab-org/gitlab#342986 (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