Add project root README

Problem/Motivation

I'm currently working on a project, where I like to parse my project's root README.md and show it to the sitebuilders. While researching if a comparable solution already exists, I stumbled across this project and saw, that it more or less does, what I had in my mind.

The only difference is, that I like to render the README.md of my project repository and provide it to the user.

Proposed resolution

While investigating, I saw, that the ReadmeManagerInterface is quite coupled to the use of modules (and not other folders). That makes it impossible to easily extend your classes, without rebuilding most of it again.

As a result I came up with a bit hacky approach, which can be used a proof of concept for an better integration. I will like to attach it here for further internal usage or when other might have the same needs.