Incorporate README into generated HTML documentation

[mattt]

Most projects provide a helpful overview in their README, and it'd be nice to include that in the generated HTML documentation. This requires a new, optional parameter for the generate command (defaulting to README.md) and additional design work.

[MaxDesiatov]

It would be great if there was an option not just to include README.md, but a given directory hierarchy of multiple markdown files that provide extended documentation: tutorials, guidelines, license, code of conduct etc.

[victor-pavlychko]

Also would be nice to have some special markdown file providing a brief introduction for the index page header.