feat(README): Add design tokens mention

mentioned in commit 7e79754b

mentioned in merge request status-page!903

mentioned in merge request gitlab!151967 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3839 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!486 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!334 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!37 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!570 (merged)