Update README.md, add SAST_PULL_ANALYZER_IMAGES

What does this MR do?

Adds documentation for SAST_PULL_ANALYZER_IMAGES variable that was previously missing.

See related MR for dependency_scanning: dependency-scanning@74710c73

cc @axil as this variable was missing from the docs, we should add it to docs.gitlab.com as well with https://gitlab.com/gitlab-org/gitlab-ee/issues/10120

What are the relevant issue numbers?

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 (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
  • Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer