code: func (s *ActivityService) MarkNotificationsRead(ctx context.Context, lastRead time.Time) (*Response, error) {
  	opts := &markReadOptions{
  		LastReadAt: lastRead,
  	}
  	req, err := s.client.NewRequest("PUT", "notifications", opts)
  	if err != nil {
  		return nil, err
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // MarkNotificationsRead marks all notifications up to lastRead as read.
  //
  // GitHub API docs: https://developer.github.com/v3/activity/notifications/#mark-as-read