Extract info from spec and put in cosmwasm/docs

[ethanfrey]

Some of this info is there already, other is a more technical deep dive.

I doubt anyone will look here for it, and we should overhaul both README.md and spec. Let's take what we can to https://github.com/cosmwasm/docs, but important tech notes next to the code (godoc, rustdoc), and remove the info that doesn't matter.