Update README.md
What does this MR do?
Adds a deprecation notice to the README.
Based on suggestion: https://gitlab.com/gitlab-org/security-products/release/-/merge_requests/137#note_1913670685
Adds a deprecation notice to the README.
Based on suggestion: https://gitlab.com/gitlab-org/security-products/release/-/merge_requests/137#note_1913670685