Document release process and required permissions

What does this MR do?

This MR documents the gemnasium release process and required permissions as discussed here

What are the relevant issue numbers?

Document gemnasium release process (gitlab-org/gitlab#408318 - 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