Improve the documentation so that newcomers understand better what GR is about

[locomundo]

We could improve the README so it's clearer for someone who doesn't know about Green Reviews to understand the project.
Also, things have changed, so we need to update the documentation and diagrams.

We had some feedback from people who wanted to use the project but it wasn't completely clear what the project does.
"A more simplified diagram would help".

[AntonioDiTuri]

This commit has improved the diagram: e2ff441
We will change again the readme once green reviews will be accepted as subproject under the operational-resilience tag.