Document custom datasets in rom-sql

[solnic]

There's no information about overridding dataset using a block, ie:

require 'rom-sql'

rom = ROM.container(:sql, 'sqlite::memory') do |config|
  config.gateways[:default].create_table :users do
    primary_key :id
    column :name, String
    column :email, String
  end

  config.relation(:users) do
    schema do
      attribute :id, ROM::SQL::Types::Serial
      attribute :name, ROM::SQL::Types::String
    end

    dataset do |rel|
      select(*rel.schema.attributes.keys).order(:id)
    end
  end
end

rom.relations[:users].insert(name: 'Jane')

puts rom.relations[:users].first.inspect