Add a philosophy document to exercises directories?

In https://github.com/exercism/x-common/pull/375#issuecomment-247817577 @petertseng says:

> these things should be documented so that we can avoid duplication of past efforts.

How about if we create a file in the exercise subdirectory that documents our discussions and efforts?

We wouldn't need to add this to all exercises, nor all at once. But as we have discussions it would be incredibly useful to be able to have the results of these discussions on hand.