Add examples to show association of const/var

I've tried to keep this in the tone of the rest of the document. There is some redundancy (Food and Weight are both used to demonstrate var association), but I think the the two different ways of declaring your package-level Food vars helps clarify the difference between `var V T = Exp` and `var V = T(Exp)` in godoc's view.

I was considering writing 'Because Favourite is declared ...' at L#53, but the double capitalisation looked ugly (and you have not done this except for Applename) so I skirted the issue by using 'it'

Also. You may want to check for spelling as American in case I forgot any words.