Improve README.md

[3100]

README should contain the purpose of this repository, a description of its structure, what it requires, and how to execute tests.
We would also like to mention the test scenario.

[AndreAugusto11]

This would be a very good help, especially for newcomers 😀

[storiumi]

Christian also suggested improve demo/readme

In the "documentation" https://github.com/datachainlab/fabric-besu-cross-demo/tree/main/demo it states make init and make network but I think they should be make build and make network (which until now it does not finishes in my enviroment (Ubuntu 22).