Updated Async Backup (markdown)

Async-Backup.md

@@ -59,7 +59,7 @@ The `weed filer.backup` process can also be stopped at any time. The current bac
 
 There are 2 backup strategies supported as of now:
 1. Continuously replicated changes. All the content are stored with the same directory structure as the source, but in a remote machine or a cloud vendor.
-2. 
+2. Differential Incremental mode. New or updated content are stored in its folder named as modified date. Deletions are not applied.
 
 ## Incremental Mode
 If `is_incremental = true`, all the files are backed up under the `YYYY-MM-DD` directories, which the timestamps are based on modified time.