Document how readme is the index for a directory

[cfjedimaster]

I'm going through the docs and while it is implied, it doesn't seem to be spelled out that the readme.md file is considered the 'index' for the site/folder. I first noticed this on the links doc (https://vuepress.vuejs.org/guide/markdown.html#links) where it shows a link to /foo/ and it's says:

<!-- Sends the user to index.html of directory foo -->

But in the file list above it, there is no index.html. I guess that would be another bug then (to go along with the general request in this issue :).

[lbenie]

It's well explained in the doc here.

Each sub-directory in your static site should contain a README.md. It will automatically be converted to index.html.

[cfjedimaster]

Dang that is super obvious - sorry!