Add note about cache when scrubbing

docs/usage.md

@@ -86,6 +86,12 @@ This command will download item data from ffxiah.com and save it to a CSV file.
 
     There is a pre-built CSV file in the repo, so you do not need to run this command if you don't want to.
 
+!!! note
+
+    Pass the  `--cache .cache` flag to the `scrub` app to save intermediate data during the scrubbing process.
+    This will allow you to resume the scrubbing process if it is interrupted.
+    Delete the `.cache` folder to start the scrubbing process from scratch.
+
 !!! note
 
     Set FFXIAHBOT_SCRUB_MAX_FAILURES=N to the number of failures (N) you are willing to accept before the scrubber stops.