Document alias level depth in README

[Spone]

I had weird issues caused by an enum defined on my model that conflicted with closure_tree. I had a hard time understanding this because this alias was not documented in the README.

class TaxonomyNode < ApplicationRecord
  enum :level, { family: 0, subfamily: 1, tribe: 2, genus: 3, subgenus: 4 }, validate: true

  has_closure_tree
end

This PR adds an mention in the README.

It would also be a good idea to provide a list of potentially conflicting method names.