Add a l3af-project level .github repo and associated files

[vmbrasseur]

To make the project friendlier for new users/contributors, we should create a top-level README.md file for the l3af-project GitHub organisation.

Here's a pointer on how to do that.

The README should, at minimum (and not necessarily in this order)…

1. Point to the CoC
2. Point to the Contributing file
3. Summarise what stuff is in which repo
4. Point to a 'getting started' guide
5. Point to the mailing list, slack, meetings, wiki

Most of the files mentioned here can exist in the .github repo then be inherited by other repos (if they aren't overridden by local files).