Added few words to docs

Author: Anton-Shutik

docs/usage.md

@@ -432,7 +432,7 @@ class LineItemViewSet(viewsets.ModelViewSet):
 #### HyperLinkedRelatedField
 
 In order to improve performance by saving some sql queries we can skip `data`
-key. Use `HyperLinkedRelatedField`. It works same as `ResourceRelatedField`
+key, and thus decrease payload size. Use `HyperLinkedRelatedField`. It works same as `ResourceRelatedField`
 but just skips `data` calculating.
 
 ### RelationshipView