Document the doc/ directory

The directory is empty except an outdated README.
So, maybe not the best user-experience for people that look for documentation and naively open the `doc/` directory...

So update the README and offers a `Makefile` to generate something in the directory.
While you are at it, why not `… visit [http://nitlanguage.org/doc/](http://nitlanguage.org/doc/)`?