Expand Readme to clairfy Auto backup style

[TWDickson]

Suggestion:

Auto backup style

Full

• Backs up all files every time.
• The backup destination is made to exactly match the current vault state, including propagating deletions (files deleted locally are removed from the backup).
• Slow

Only new

• Backs up only files that are new or have changed since the last backup.
• Still propagates deletions: if a file is deleted locally, it is also removed from the backup so the backup mirrors the current vault state.

Non-destructive

• Same as Only new for new/changed files (only uploads differences).
• Does not delete anything from the backup: files deleted locally are kept in S3.
• The backup becomes an append-only archive of everything that has ever been backed up, not a point-in-time snapshot of the vault.