Add ReadOnlyField to TokenRefreshSerializer (#396)

To make TokenRefreshSerializer be more descriptive, by adding access field as a ReadOnlyField()

rest_framework_simplejwt/serializers.py

@@ -94,6 +94,7 @@ def validate(self, attrs):
 
 class TokenRefreshSerializer(serializers.Serializer):
     refresh = serializers.CharField()
+    access = serializers.ReadOnlyField()
 
     def validate(self, attrs):
         refresh = RefreshToken(attrs['refresh'])