update readme + upgrade

Author: Babichev Maxim

README.md

@@ -98,7 +98,7 @@ class Item extends Model implements Product
         return true; 
     }
 
-    public function getAmountProduct(): int
+    public function getAmountProduct(Customer $customer): int
     {
         return 100;
     }
@@ -107,8 +107,7 @@ class Item extends Model implements Product
     {
         return [
             'title' => $this->title, 
-            'description' => 'Purchase of Product #' . $this->id, 
-            'price' => $this->getAmountProduct(),
+            'description' => 'Purchase of Product #' . $this->id,
         ];
     }
 

docs/ru/upgrade-guide.md

@@ -63,3 +63,31 @@ class Item extends Model implements Product
 ## 3.0.x → 3.1.x
 
 Замените `Taxing` на `Taxable`.
+
+## 3.1.x → 4.0.x
+
+> Если вы используете php 7.1, то версия 4.0 вам не доступна. 
+> Вам необходимо обновить php.
+
+Удалили поддержку старых версий `laravel/cashier`. Поддержка начинается от 7+.
+
+#### Если используете оплаты
+
+Вам необходимо добавить аргумент `Customer $customer` в метод `getAmountProduct` 
+вашей модели.
+
+Ваш код на 3.x:
+```php
+    public function getAmountProduct(): int
+    {
+        return $this->price;
+    }
+```
+
+Ваш код на 4.x:
+```php
+    public function getAmountProduct(Customer $customer): int
+    {
+        return $this->price;
+    }
+```

docs/upgrade-guide.md

@@ -63,3 +63,31 @@ class Item extends Model implements Product
 ## 3.0.x → 3.1.x
 
 Replace `Taxing` to `Taxable`.
+
+## 3.1.x → 4.0.x
+
+> If you are using php 7.1, then version 4.0 is not available to you. 
+> You need to update php.
+
+Removed support for older versions of `laravel/cashier`. We support 7+.
+
+#### If you use payment for goods
+
+You must add the argument `Customer $customer` to the `getAmountProduct` 
+method of your model.
+
+Your code on 3.x:
+```php
+    public function getAmountProduct(): int
+    {
+        return $this->price;
+    }
+```
+
+Your code on 4.x:
+```php
+    public function getAmountProduct(Customer $customer): int
+    {
+        return $this->price;
+    }
+```