Enhance yaml-xml serialization groups configuration doc

Author: GregoireHebert

core/getting-started.md

@@ -196,6 +196,26 @@ resources:
             pagination_items_per_page: 25    # optional
 ```
 
+And tell your configuration how to load it:
+
+```yaml
+# api/config/packages/api_platform.yaml
+api_platform:
+    mapping:
+        paths: ['%kernel.project_dir%/config/api_platform']
+```
+
+Note that the resources configuration only support the normalization / denormalization context definition in `yaml` and `xml`. To configure the groups of serialization for each attribute, you need to define your groups like you would usually on a standard Symfony project. See the [Symfony doc](https://symfony.com/doc/current/components/serializer.html#attributes-groups).
+
+You might also need to update your framework configuration with this:
+
+``` yaml
+# api/config/packages/framework.yaml
+framework:
+    serializer:
+        enabled: true
+```
+
 **You're done!**
 
 You now have a fully featured API exposing your entities.