Explain automatic folder updates in the user guide

README.md

@@ -140,7 +140,9 @@ Once searchi is running:
 
 How quickly a folder becomes searchable depends on its size and contents.
 The desktop keeps its search model warm for subsequent searches.
-Use **Reindex** after adding or changing files in an indexed folder.
+searchi watches your chosen folders and updates search in the background as
+files change. You can also use **Reindex** to check a folder again, for example
+after reconnecting an external drive.
 
 searchi automatically chooses supported graphics acceleration when available
 and falls back to CPU when it cannot use it. You do not need a graphics