code: def add_family(self, major_number):
          
          # Normally, we have separate buckets for bugfixes vs features
          keys = ['unreleased_bugfix', 'unreleased_feature']
          # But unstable prehistorical releases roll all up into just
          # 'unreleased'
          if major_number == 0 and self.config.releases_unstable_prehistory:
              keys = ['unreleased']
          # Either way, the buckets default to an empty list
          self[major_number] = {key: [] for key in keys}
docs: Expand to a new release line with given ``major_number``.
  
          This will flesh out mandatory buckets like ``unreleased_bugfix`` and do
          other necessary bookkeeping.