WIP: doc update

[apatard]

I've started some time ago to update the documentation, and for now, it looks like: #172.

Before merging, some feedback would be nice, as in:

• what's possibly missing,
• what can be improved in the current PR.

Please share.

[jimbo8098]

I'd like to see a newer version of Ubuntu added to the README personally but I know your reasons for not including (discussed in #97 ). I don't think it's wise to rely on CI providers giving access to nested virtualization so I think it's better to just put together a test lib that people can run on their own builds. That kind of relies on trust though. Maybe I could build something in Packer to prove it works on Ubuntu 20.04+?