Split repo, crate and possibly website readme

[robo9k]

Right now the same README.md is used for the Git repo and the published crate. The GitHub Pages website only consists of rustdoc.

This is also required/helpful for some of #48

[robo9k]

Instead of a full-blown website, maybe another markdown document will suffice: https://doc.rust-lang.org/rustdoc/unstable-features.html#--index-page-provide-a-top-level-landing-page-for-docs