Rework the README

I split the `README` into two files:

 * `CONTRIBUTING.adoc`
 * `README.adoc`

I removed the part about "Using Asciidoctor and the native JavaScript environment from Ruby". I think it's an advanced topic, maybe we can add a wiki to explain how to do it ? Or a new file `GUIDE.adoc` ?
We also need to add a topic on how to write extension in JavaScript and how to build Asciidoctor options with `Opal.hash2`.