docs(content-negociation): document how to declare an encoder for custom format

Author: nikophil

core/content-negotiation.md

@@ -200,6 +200,19 @@ API Platform will automatically call the serializer with your defined format nam
 It will then return the result to the client with the requested MIME type using its built-in responder.
 For non-standard formats, [a vendor, vanity or unregistered MIME type should be used](https://en.wikipedia.org/wiki/Media_type#Vendor_tree).
 
+You will also need to declare an encoder which supports the new format:
+
+```yaml
+services:
+    app.api-platform.myformat.encoder:
+      class: ApiPlatform\Serializer\JsonEncoder
+      argumments:
+          $format: 'myformat'
+      # The following lines are only needed if autoconfigure is disabled
+      # tags:
+      #     - { name: 'serializer.encoder' }
+```
+
 ### Reusing the API Platform Infrastructure
 
 Using composition is the recommended way to implement a custom normalizer. You can use the following template to start your