Update Chargebee guide with a note about control fields

guides/chargebee.mdx

@@ -139,6 +139,10 @@ Finally, the following keys can be used to control the importer's default behavi
 | `invopop-workflow` | Workflow to use for this Customer (overrides the default one) | `cf_invopop_workflow` |
 
 <Tip>
-You would typically use either `invopop-include` to allowlist the customers to be imported, or `invopop-exclude` to denylist them, but not both. Both are optional, though, and if not set, all customers' invoices will be imported.
+You would typically use either `invopop-include` to allowlist the customers to be imported, or `invopop-exclude` to denylist them, but not both. They are optional, though, and if not set, all customers' invoices will be imported.
 </Tip>
 
+<Warning>
+When you configure a new custom field in Chargebee, it is not immediately added to any preexisting entities. The custom field is only attached to an entity after you edit and save it. This is particularly relevant to the `cf_invopop_include` field. The importer will always import a Customer unless the field is present and set to "False". To ensure the expected behaviour, you should probably [back-populate the field in bulk](https://www.chargebee.com/docs/2.0/bulk-operations.html) for all your Customers to "False" right after configuring the field.
+</Warning>
+