Added all SEPA countries (IBANs from European Union countries) and minimal test coverage for countries

lib/iban_ex/country/de.ex

@@ -1,6 +1,13 @@
 defmodule IbanEx.Country.DE do
   @moduledoc """
   Germany IBAN parsing rules
+
+  ## Examples
+
+    iex> %IbanEx.Iban{country_code: "DE", check_digits: "89", bank_code: "37040044", branch_code: nil, national_check: nil, account_number: "0532013000"}
+    iex> |> IbanEx.Country.DE.to_string()
+    "DE 89 37040044 0532013000"
+
   """
 
   @size 22