Add authorized file-root sync

README.md

@@ -108,7 +108,9 @@ The bootstrap implementation provides:
   `unpin`, `cleanup`, `providers`, `list`; remote fetch accepts
   `--bearer-secret <secret>`
 - local CAS tree objects describe file trees and are stored as CAS blobs
-- local file-root commands: `geth cas root add/list/scan`
+- local file-root commands: `geth cas root add/list/scan/sync`; root sync pulls
+  authorized remote tree metadata and CAS tree bytes into a peer-qualified
+  remote root without writing files
 - local file conflict metadata commands:
   `geth cas conflict record/list/resolve`
 - local DB resource registration: `geth db add <name> <path>` and