Add a README file

The README is the start of some helpful documentation, but it's also being used to mark the transition to gitlab as the canonical source for lego. The discussion about that will happen in the MR for this.

@lavamind, @HackerNCoder, @gus, @duncan, @hiro CC'ing all of you in since you seem to be the biggest stake holders for lego. Lavamind and I realized that there's no real channel to announce something like this, so this seemed like a much more visible way to do that.

The gist of it is that gitolite is making things harder with lego, so we'd like to move the canonical source of lego to gitlab. Gitolite is deprecated, all the lektor sites are on gitlab, and the gitolite repo has HEAD pointing to a non-existant branch (master) for some reason, so it's breaking checkouts and CI.

If this change goes through, we'll need to go through all the lego sites and update the .gitmodules files (team#23 (closed))

What do y'all think about this? Are there any concerns about the move, or reasons we shouldn't move?