Improve description

schemas/commodity-README.md

@@ -53,7 +53,8 @@ In the list of commodites:
 
 If the data is sourced from the journal folder:
 - Remove the `$` prefix and `_name;` suffix from the `Name` field.
-- There will be no `prohibited` and `economies` to send. Exclude these from the message.
+- As the Journal Market.json doesn't contain `economies` or `prohibited` data, 
+  leave these entirely out of the message.  You **MUST NOT** send empty lists.
 
 #### Item Category
 Remove not only the `Category_Localised` key:values, but also the