Idea: Show some default docs.

I ran `documentjs`, then `serve docs` and the page is just a blank index page, and I also don't see any pages that contain the documentation of the source files.

A nice default would be to show the package's README.md for the index page, and in the sidebar show links to pages where each page is the docs for a source file.

Try out [`dox-foundation`](https://github.com/punkave/dox-foundation) and [`doxx`](https://github.com/FGRibreau/doxx) to get an idea of what I mean.