[travelScreen/shop] explain consumption rate of fuel & food

[7yl4r]

"How do I know how much to buy if I don't understand the consumption rate?"

Some explanation needs to be given on this, and some sense of how far one can travel given a set amount of food and fuel would be helpful.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/30101860-travelscreen-shop-explain-consumption-rate-of-fuel-food?utm_campaign=plugin&utm_content=tracker%2F13859664&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F13859664&utm_medium=issues&utm_source=github).

[7yl4r]

should be implemented using similar methods as #108