README - rewrite the README

[bobtfish]

Make it actually attractive and informative.

Tell people how to build the project, how to install it and use it (as well as how to run the tests). (This may need some additional scaffolding to be able to install the binary we produce etc).

Split the dev notes out into a separate file.

Generally make this into an open source project that looks like something that someone might be able to use, rather than a random lump of code.

[khvzak]

I plan to integrate this project into mlua to provide a high level unified API.

For now I'll remove most of the details in README as the current state is WIP and not ready for anybody use or announcements.