Move the roadmap into TODO.md and fix a test comment

The rewrite removed the README's roadmap, which TODO.md linked to. The
roadmap items are now under Future Steps in TODO.md, in place of the
link.

A comment in internal/storage/s3_test.go cited the README for the
destination layout. That layout is in docs/REPOSTRUCTURE.md.

Model: fable-5-1

internal/storage/s3_test.go

@@ -161,7 +161,7 @@ func TestS3ListingCutShortIsAnError(t *testing.T) {
 }
 
 // TestS3URLPrefixKeyLayout pins the bucket keys an s3:// URL reads and
-// writes: the README's remote storage layout, with the prefix joined to
+// writes: the layout in docs/REPOSTRUCTURE.md, with the prefix joined to
 // each key by one "/". s3://b/p and s3://b/p/ must be the same
 // destination, or a host that writes the URL the other way finds no
 // snapshots. The listed object is put straight into the bucket, as