Document the repository as legacy

Summary:
As we switched secrets managements from Phabricator passphrase
to HashiCorp Vault, we don't use this anymore at Nasqueron.

AFAIK, this is a in-house project, used only internally,
so there is no other stakeholder. As such, it can be archived.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2856