Initial Trade Commodities & Goods

The section on Trade was eluding me for a while, but I have something of an initial pass at it now.  Along with it, I started to put together some thoughts about the trade system, but also somewhat applicable to the wider game idea behind this; these are appended as Notes at the end of the Trade Commodities & Goods document.

Between this section and the Weather section, there's a very rough system in place that would let you work out ship travels and trade in ports.  It is by no means a complete or working system, but it has come along better than I expected.