Add expression engine reference

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/164229



Merged-by: Marcel Amirault <mamirault@gitlab.com>
Approved-by: Avielle Wolfe <awolfe@gitlab.com>
Approved-by: Marcel Amirault <mamirault@gitlab.com>
Co-authored-by: Mark Nuzzo <mnuzzo@gitlab.com>