Shipyard Tax income

Give the major cities which have shipyards unique a tax income bonus. It makes sense as they have the naval strength to defend their trade routes. This is to give canon cities much higher value.

This would include :
Oldtown
The Arbor
Lannisport
Kingslanding
Driftmark
All of the eastern shipyards

Castle shipyards should exempt from this or provide a much lower bonus. Perhaps 0.5 per level.