[CreditMemo] Add notes to UPGRADE file

UPGRADE.md

@@ -1,7 +1,19 @@
+### UPGRADE FROM 1.0.0-RC.1 TO 1.0.0-RC.2
+
+1. `CreditMemoUnit` changed to `LineItem` which is a resource entity now
+
+2. `CreditMemoUnitGeneratorInterface` changed to `LineItemsConverterInterface` and moved from `Generator` to `Converter` directory
+
+3. `OrderItemUnitCreditMemoUnitGenerator` changed to `LineItemsConverter` and moved from `Generator` to `Converter` directory
+
+4. `ShipmentCreditMemoUnitGenerator` changed to `ShipmentLineItemsConverter` and moved from `Generator` to `Converter` directory
+
+There are no migrations that provide backward compatibility, save current credit memos before upgrading the version of plugin. 
+
 ### UPGRADE FROM 0.10.1 TO 1.0.0-RC.1
 
 1. `OfflineRefundPaymentMethodsProvider` renamed to `SupportedRefundPaymentMethodsProvider` with the supported gateways array as the 2nd argument
-(by default only `offline` gateway is passed and therefore supported). 
+(by default only `offline` gateway is passed and therefore supported).
 
 ### UPGRADE FROM 0.8.0 TO 0.9.0