Provide a more gentle introduction to the pedestal.

I've started to work on a `README.md` that links all of the samples, explains what they're about and has a few more notes for those wanting to learn pedestal.

Personally, I've tried to get into it a few times now, but it was a bit too much at once, I think. Especially when starting with the `chat` example.
So I've tried to give an overview about what kind of examples are there and also hint at their difficulty level.

But before I get too far into this, I'd like to know whether you want this kind of documentation, so tell me your opinion.