Adding an optional design.md document to exercise specifications

Throughout an exercise's history we often see the same discussions crop up over and over.

For example, in `meetup`, people want to add tests for `fifth` (e.g. _The fifth Monday of a month_).

This has been discussed a number of times, with the conclusion that we will not add logic for `fifth`, since the exercise is about scheduling regular meetups, and not all months have fifth weeks, which means that scheduling a meetup on the fifth Monday (or whatever) would mean that the meetup no longer met regularly.

In order to make it easier to avoid having the same discussions over and over again, we're going to add a `design.md` file to exercises where we can summarize the decisions.

**This file is optional.** We will add them as it makes sense. We chose the name `design.md` because we have a `design.md` document for the concept exercises, and the name makes enough sense in this context that we don't think it would be misleading or confusing.

If you can see a reason to do this differently, please raise your objections here! If not, we'll go ahead with this new scheme in a couple of weeks.