Added doc string to HyperlinkedMixin

Author: Anton-Shutik

rest_framework_json_api/relations.py

@@ -116,6 +116,12 @@ def get_links(self, obj=None, lookup_field='pk'):
         })
         self_link = self.get_url('self', self.self_link_view_name, self_kwargs, request)
 
+        """
+        Assuming RelatedField will be declared in two ways:
+        1. url(r'^authors/(?P<pk>[^/.]+)/(?P<related_field>\w+)/$', AuthorViewSet.as_view({'get': 'retrieve_related'}))
+        2. url(r'^authors/(?P<author_pk>[^/.]+)/bio/$', AuthorBioViewSet.as_view({'get': 'retrieve'}))
+        In other words if related_link_url_kwarg == 'pk' it will add 'related_field' parameter to reverse()
+        """
         if self.related_link_url_kwarg == 'pk':
             related_kwargs = self_kwargs
         else: