Add note about outside collaborators for cost center (#58521)

Co-authored-by: Sophie DeRosa <[email protected]>

Author: jc-clark

content/billing/concepts/cost-centers.md

@@ -35,3 +35,4 @@ For more details, see [AUTOTITLE](/billing/reference/cost-center-allocation).
 * The maximum number of resources per cost center is 10,000.
 * A maximum of 50 resources can be added to or removed from a cost center at a time.
 * Azure subscriptions can only be added to or removed from cost centers through the UI.
+* Outside collaborators can only be added to cost centers via the cost center API. For more information, see [AUTOTITLE](/billing/tutorials/control-costs-at-scale#add-resources-to-the-cost-center).