Add custom field for open-ended text at top

[cohml]

I provide my clients with multiple options for payment, but there's no great way to do this.

So instead I co-opt the ship_to field, but this is not elegant and the results are quite ugly, especially when that field must do double duty with "Payment Options" AND "Ship To".

It would be wonderful and serve many purposes beyond simply "Payment Options" if we could just slap an entirely new open-ended text field in that same area of the invoice.

Bonus points if this field can do things like support bullets, or more generally something like Markdown formatting.