Additional notes field for subscriptions

We miss a very important feature for subscriptions. It is the "Additional notes" field, that is available for invoices normally. Actually when a subscription creates an invoice, the field is there - it is just impossible to set it up upfront.

This is a big problem, because we need to set up payment instructions per-customer (they have different languages and the Appearance > Invoice Footer field is not multi-language).

It should be an easy improvement, and would help us tremendously. Right now we can not use the subscription feature solely because of this missing piece.