Add top-level README to explain briefly archive/ and meta/ behavior

This could especially be useful once #2771 (closed) is implemented.

---

Migrated from T2775 (view on Phabricator)

mentioned in merge request !70 (closed)

added Software Heritage filesystem priority:Normal labels

added priority:Low label and removed priority:Normal label

mentioned in commit ef5806f5

added state:wip label

assigned to @haltode

removed state:wip label

closed