README: Add documentation

README.md

@@ -9,4 +9,16 @@ Library for making Bankgirot supplier payment files, _Leverantörsbetalningar_.
 * På svenska: ["Leverantörsbetalningar, teknisk manual"](http://www.bgc.se/globalassets/dokument/tekniska-manualer/leverantorsbetalningar_tekniskmanual_sv.pdf) (PDF)
 * In English: ["Supplier Payments (Leverantörsbetalningar), technical manual"](http://www.bgc.se/globalassets/dokument/tekniska-manualer/supplierpayments_leverantorsbetalningar_technicalmanual_en.pdf) (PDF)
 
+Records are defined in `lib/supplier_payments/payment_file/domestic_records.rb`. Layouts are defined with arrays, like:
+
+```ruby
+[ :credit_transfer_number, 6, 'N', :right_align, :zerofill ]
+# :credit_transfer_number is the name of the field
+# 6 is the length of the field
+# `"N"` if the value consists of two digits, or `"A"` for strings.
+# Options for formatting are:
+#   :right_align if the field is to be right-aligned (otherwise it will be left-aligned)
+#   :zerofill if the field is to be filled with zeros
+```
+
 [API documentation for this gem](https://www.rubydoc.info/gems/supplier_payments/)