Update readmes / descriptions of core spec / org / key repos to be more welcoming

[cholmes]

Ahead of an announcement we should be sure that we explain a bit more about the project to new users. Matthias has made a great start on this, but a few more things would be good to have:

• [x] Update fiboa org readme so it's a great landing page for the project. It should include a clear mission statement, key features, development status, license info, communication options (slack, zoom meeting, etc).
• [x] Update spec readme, to include at least a subset of the above, and to make clear the status of the spec and more general readme info for people to get a sense of things.
• [x] Update repo 'descriptions' - most repos in Fiboa right now have no 'about'. They should ideally include those, and any other good meta information.
• [x] Update other key readmes - check to be sure the most prominent readmes are all in good shape (validator, cli, ai extension, etc)
• [ ] Make sure instructions at top level are all in line with the way we want people to work.

[m-mohr]

@cholmes What is meant with "Make sure instructions at top level are all in line with the way we want people to work."?

[m-mohr]

Closing for now, let me know if you think there's more to do @cholmes