Add a note about versioning of client vs API spec

Author: bradymholt

README.md

@@ -80,6 +80,10 @@ The following methods are available in this library.
 |                            | [scheduled_transactions.get_scheduled_transaction_by_id(budget_id, scheduled_transaction_id)](https://github.com/ynab/ynab-sdk-ruby/blob/master/docs/ScheduledTransactionsApi.md#get_scheduled_transaction_by_id) | Returns a single scheduled transaction                |
 |                            | [scheduled_transactions.create_scheduled_transaction(budget_id, data)](https://github.com/ynab/ynab-sdk-ruby/blob/master/docs/ScheduledTransactionsApi.md#create_scheduled_transaction)                           | Creates a single scheduled transaction                      |
 
+## Versioning
+
+The version of this client is defined in the `ynab.gemspec` file and follows [semantic versioning](https://semver.org/).  The version of this client is maintained independently and does not align with the the version of YNAB API itself (which is defined in the [OpenAPI spec](https://api.ynab.com/papi/open_api_spec.yaml)).  To determine which spec version of the YNAB API was used when generating this client you can refer to the "description" in the  field in the `ynab.gemspec` file.
+
 ## License
 
 Copyright (c) 2024 You Need A Budget, LLC