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.

[kulikthebird]

• Removal of the spec folder is done here: docs: Move relevant info from spec to cosmwasm documentation #566
• Adding any information that is relevant from spec to CW docs: Move wasmvm/spec and cosmwasm/SEMANTICS.md here docs#167

[kulikthebird]

The spec/ directory was removed from this repository. In my opinion the information there consist of:
- outdated plans / initial designs,
- discussions about concepts and further steps,
- things that are already well documented somewhere else.