Update deprecation.md

Add yaml notation  for single property deprecation

Author: jzabrodin

core/deprecations.md

@@ -98,6 +98,19 @@ class Review
 }
 ```
 
+```yaml
+# api/config/api_platform/resources.yaml
+resources:
+    # ...
+    App\Entity\Review:
+        properties:
+            # ...
+            letter:
+                attributes:
+                    deprecation_reason: 'Use the rating property instead'
+                    
+```
+
 * With JSON-lD / Hydra, [an `owl:deprecated` annotation property](https://www.w3.org/TR/owl2-syntax/#Annotation_Properties) will be added to the appropriate data structure
 * With Swagger / OpenAPI, [a `deprecated` property](https://swagger.io/docs/specification/2-0/paths-and-operations/) will be added
 * With GraphQL, the [`isDeprecated` and `deprecationReason` properties](https://facebook.github.io/graphql/June2018/#sec-Deprecation) will be added to the schema