WIP draft of currency-exchange concept exercise

[colinleach]

Pairs with the numbers concept.

The .docs/introduction.md will need to be copied from numbers/introduction.md.

Also, .meta/design.md still needs info on which concepts it unlocks, once we decide this.

[colinleach]

I've added an introduction.md, which is a small fraction of the concept's about.md. I may cut this down further: Jeremy is very insistent that only things directly relevant to the exercise should be included.

I also filled in the rest of .meta/design.md with a tentative list of which concepts it unlocks. This may not survive later developments.

I'll mark this as "Ready for review", but don't rush into doing much. Apart from my doubts around the introduction, we may avoid complaints from the (over-eager?) linter if we merge the concept (numbers) before the exercise. It's a config.sys thing.