Add repo-level README

[hammer]

It would be nice to have some documentation of the aims and scope of this project in a repo-level README. I'm not in a big hurry though, since there's no code yet!

I think some combination of our design principles and the core operations we intend to support would be a good content to include.

[hammer]

Oh and a link to our Discourse, which we may want to relocate to forums.sgkit.org or something.