Add config example for class_serializers

Author: Alejandro Perez

README.md

@@ -201,6 +201,20 @@ final class SentryCustomizationCompilerPass implements CompilerPassInterface
 }
 ```
 
+#### Custom serializers
+
+The option class_serializers can be used to send customize objects serialization.
+```yml
+sentry:
+    ...
+    options:
+        ...
+        class_serializers:
+            SerializableClassName: '@YourSerializerService'
+```
+
+Several serializer can be added and the serializable check is done using instanceof. The serializer must implements the `__invoke` method returning an **array** with the information to send to sentry (class name is always sent).
+
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