Updated docs with support for idempotency_keys

docs/history/2_8_0.md

@@ -21,6 +21,7 @@
 - `djstripe.signals.webhook_processing_error` now also takes `instance` and `api_key` arguments
 - `stripe.api_version` is no longer manipulated by dj-stripe.
 - Resolved ambiguity between `LineItem` and `InvoiceItem` models. It was incorrectly assumed that the `lines` List object on `Invoice` and `UpcomingInvoice` models only return `InvoiceItem` objects. Moreover `LineItem` objects can also be of type `subscription` if the user adds a Subscription to their `Invoice` as a lineitem.
+- Added support for `idempotency` keys. `djstripe` now automatically generates and puts keys in Stripe Object's Metadata (if possible). Users have the option to supply their own as well for `create` calls.
 
 ## Deprecated features
 
@@ -46,7 +47,6 @@
  created `Refund` object is now returned as opposed to
  the `Charge` object.
 - Renamed `get_idempotency_key` to `create_idempotency_key` to better reflect its functionality.
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 ## Other changes