Add description of how to update custom claims on refresh to README

Author: henrywoody

README.rst

@@ -276,6 +276,9 @@ SLIDING_TOKEN_REFRESH_EXP_CLAIM
 Customizing token claims
 ------------------------
 
+Adding custom claims
+~~~~~~~~~~~~~~~~~~~~
+
 If you wish to customize the claims contained in web tokens which are generated
 by the ``TokenObtainPairView`` and ``TokenObtainSlidingView`` views, create a
 subclass for the desired view as well as a subclass for its corresponding
@@ -309,6 +312,49 @@ for the view, which is in turn used to generate the view's access token.
 As with the standard token views, you'll also need to include a url route to
 your subclassed view.
 
+Refreshing custom claims
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Certain properties of a token may be updated during the lifetime of a refresh
+token. For example, if a user's name is included in the token and the user is
+allowed to update their name, access tokens should be updated to reflect these
+changes. If you wish to send updated custom token claims with each access
+token, which are generated by the ``TokenRefreshView`` and the
+``TokenRefreshSlidingView``, create a subclass of the desired view as well as a
+subclass of its corresponding serializer. Here's an example of how to customize
+the claims in tokens generated by the ``TokenRefreshView``:
+
+.. code-block:: python
+
+  from django.utils.six import text_type
+  from rest_framework_simplejwt.serializers import TokenRefreshSerializer
+  from rest_framework_simplejwt.views import TokenRefreshView
+  from rest_framework_simplejwt.tokens import AccessToken
+  from my_auth_app.models import User   # custom user model
+
+  class MyTokenRefreshSerializer(TokenRefreshSerializer):
+      def validate(self, attrs):
+          # get default access token
+          data = super().validate(attrs)
+          access_token_str = data.get('access')
+          access_token = AccessToken(access_token_str)
+
+          # get user for token
+          user = User.objects.get(pk=access_token['user_id'])
+
+          # update custom fields
+          access_token['name'] = user.name
+          # ...
+
+          data['access'] = text_type(access_token)
+          return data
+
+  class MyTokenRefreshView(TokenRefreshView):
+      serializer_class = MyTokenRefreshSerializer
+
+As with the standard token views, you'll also need to include a url route to
+your subclassed view.
+
 Token types
 -----------