fix: upgrade guide for 3.4

Author: soyuka

core/upgrade-guide.md

@@ -1,5 +1,52 @@
 # Upgrade Guide
 
+## API Platform 3.4 
+
+Remove the `keep_legacy_inflector`, the `event_listeners_backward_compatibility_layer` and the `rfc_7807_compliant_errors` flag:
+
+```diff
+api_platform:
+-        event_listeners_backward_compatibility_layer: false
+-        keep_legacy_inflector: false
+        extra_properties:
+            standard_put: true
+-            rfc_7807_compliant_errors: true
+```
+
+If you use a custom normalizer for validation exception use:
+
+```yaml
+api_platform:
+	validator:
+		legacy_validation_exception: true
+```
+
+Indeed, we will throw another validation class in API Platform 4 we will throw `ApiPlatform\Validator\Exception\ValidationException` instead of `ApiPlatform\Symfony\Validator\Exception\ValidationException`
+
+It's really important to add the `use_symfony_listeners` flag, set to `true` if you use Symfony listeners or controllers:
+
+```yaml
+api_platform:
+	use_symfony_listeners: false
+```
+
+The `keep_legacy_inflector` flag will be removed from API Platform 4, you need to fix your issues first. In API Platform 3.4, the Inflector is available as a service that you can configure through:
+
+```yaml
+api_platform:
+    inflector: api_platform.metadata.inflector
+```
+
+Implement the `ApiPlatform\Metadata\InflectorInterface` if you need to tweak its behavior.
+
+We added an `hydra_prefix` configuration as the `hydra:` prefix will be removed by default in API Platform 4:
+
+```yaml
+api_platform:
+    serializer:
+        hydra_prefix: false
+```
+
 ## API Platform 3.1/3.2
 
 This is the recommended configuration for API Platform 3.2. We review each of these changes in this document.