Add builtins example to README

There seem to be a bunch of questions about how to do this and people keep saying "I figured it out" without telling anyone else how they did it!  This is how you can do it, in the README!

README.rst

@@ -104,12 +104,21 @@ To do so you should override ``JSONFormatter.json_record()``.
 .. code-block:: python
 
     class CustomisedJSONFormatter(json_log_formatter.JSONFormatter):
-        def json_record(self, message, extra, record):
+        def json_record(self, message: str, extra: dict, record: logging.LogRecord) -> dict:
             extra['message'] = message
             extra['user_id'] = current_user_id()
             extra['ip'] = current_ip()
+            
+            # Include builtins
+            extra['level'] = record.levelname
+            extra['name'] = record.name
+            
             if 'time' not in extra:
                 extra['time'] = django.utils.timezone.now()
+            
+            if record.exc_info:
+                extra['exc_info'] = self.formatException(record.exc_info)
+        
             return extra
 
 Let's say you want ``datetime`` to be serialized as timestamp.