[Tagging] Builders' Merchants -> Timber Merchant

[ael]

> would be useful. When I have a moment, I will try and come up with
> something better than yard.

A thesaurus didn't throw up anything compelling. Instead I became
convinced that the best subtag is actually "merchant".

shop=merchant
  merchant = timber|building-supplies|agricultural|garden|...

I realise that "merchant" is a less familiar word in some parts of the
world, but the values should make it obvious. I suppose that
"distributer" might be an alternative but I don't think it is the same.

Craig's shop=industrial_supplies also seems to be to be valueable and
to capture something a bit different, so I suggest that also be
introduced. I guess there is some risk of overlap, but I don't see why
would be a problem.

shop=industrial_supplies
 industrial_supplies = chemical|construction|metal|...

Reactions? Should I (or someone else) put something on the wiki?
At least if we have reached a consensus...

ael

PS. Yes I suppose one could argue that every shop is a merchant, and
it should be the main tag with shop as a subtag, but we have to live 
with the status quo.