Add an example in the repo README.md file

[Ogeon]

Description

There have been requests for some sort of code example for those who find the repo and would like to see how it looks and works. I think one or two examples can be added, together with the link to the inner readme. It should be automatically tested, just like the crate readme.

Motivation

Gives a quick preview of the library for those who don't want to dig into the sub-directories.