Add weight calculation documentation

[SWeini]

Planning tools that work only on data stage (e.g. YAFC) need a way to replicate the weight calculation, but the little information given is not enough to explain what is going on.

Example that might involve some rounding
electronic-circuit
1x iron plate (1000)
3x copper cable (250 each)
ingredient_to_weight_coefficient 0.28
---
1750 * 0.28 = 490
but in-game weight is 500

Example that might involve the fact that the item can be placed as entity
wooden-chest
2x wood (2000 each)
ingredient_to_weight_coefficient 0.5
---
4000 * 0.5 = 2000
but in-game weight is 20000 (yes, that's off by an order of magnitude)

I have really no idea what is going on, but need it for cross-surface transportation optimization. Please add detailed information to the auxiliary section.

[thesixthroc]

+1