Update README with a warning on limitations of fetch

README.md

@@ -51,6 +51,7 @@ Object IDs are auto-incremented, and correspond to the order the entry were
 inserted onto the system.
 
 ## Fetching Records
+> :warning: **Limitation! Fetch can only fetch limited classes correctly**: int, float and str!
 Finally, you may wish to recreate objects from a table that already exist, for
 this purpose we have the function `fetch_from(class_, object_id)` as well
 as `is_fetchable(className, object_id)` former fetches a record from the