Document new serialize columns

[jwoertink]

luckyframework/avram#695

[jwoertink]

Just a few notes on how to use this:

class User < BaseModel
  table do
    column preferences : UserPreferences, serialize: true
  end
end

struct UserPreferences
  include JSON::Serializable
  property? receive_email : Bool = true
end

class SignUpUser < User::SaveOperation
  before_save do
    preferences.value = UserPreferences.from_json("{}")
  end
end

class UpdateUser < User::SaveOperation
  attribute allow_email : Bool
  before_save do
    prefs = preferences.value.not_nil!
    prefs.receive_email = !!allow_email.value
  end
end

user = UserQuery.new.first
user.preferences.receive_email?

As of right now, there's no special way to query against it.