Add note about getters

UPGRADE.md

@@ -40,6 +40,31 @@
         }
     ```
 
+1. The getter methods of `Sylius\RefundPlugin\Entity\CustomerBillingData` has been changed:
+
+    ```diff
+    -   public function id(): ?int
+    +   public function getId(): ?int
+    -   public function firstName(): string
+    +   public function getFirstName(): string
+    -   public function lastName(): string
+    +   public function getLastName(): string
+    -   public function fullName(): string
+    +   public function getFullName(): string
+    -   public function street(): string
+    +   public function getStreet(): string
+    -   public function postcode(): string
+    +   public function getPostcode(): string
+    -   public function city(): string
+    +   public function getCity(): string
+    -   public function company(): ?string
+    +   public function getCompany(): ?string
+    -   public function provinceName(): ?string
+    +   public function getProvinceName(): ?string
+    -   public function provinceCode(): ?string
+    +   public function getProvinceCode(): ?string
+   ```
+
 ### UPGRADE FROM 1.0.0-RC.9 TO 1.0.0-RC.10
 
 1. Support for Sylius 1.8 has been dropped, upgrade your application to [Sylius 1.9](https://github.com/Sylius/Sylius/blob/master/UPGRADE-1.9.md)