add a note on how to upgrade

UPGRADING.md

@@ -1,3 +1,55 @@
+From 2.2.3 to 2.3.0
+====================
+
+- The `\Serializable` PHP interface is deprecated, the methods of this interface will be removed in 3.0.
+  This change is done to allow the use of the new `__serialize` and `__unserialize` PHP's strategy.
+
+  If you were extending the metadata classes, your custom serialization methods were looking probably as something as this:
+
+    ```php
+    class MyMetadata extends PropertyMetadata 
+    {
+        // ... more code
+
+        public function serialize()
+        {
+            $data = parent::serialize();
+
+            return \serialize([$data, $this->customMetadataProp]);
+        }
+
+        public function unserialize($str)
+        {
+            list($str, $this->customMetadataProp) = \unserialize($str);
+
+            parent::unserialize($str);
+        }
+    }
+    ```
+
+    After this change, your code should look like this:
+
+    ```php
+    class MyMetadata extends PropertyMetadata 
+    {
+        // ... more code
+
+        protected function serializeToArray(): array
+        {
+            $data = parent::serializeToArray();
+
+            return [$data, $this->customMetadataProp];
+        }
+
+        protected function unserializeFromArray(array $data): void
+        {
+            list($data, $this->customMetadataProp) = $data;
+
+            parent::unserializeFromArray($data);
+        }
+    }
+    ```
+
 From 1.7.0 to 2.0.0
 ====================