Add & update appendix
I think we should have a version of the appendix that's in sync with the Coq development right in the repository. The latest public version is outdated, and I have sent various snapshots of the appendix to various people. Pointing them to this git repository would make it possible for them to stay in sync with updates, while still making it clear that this is all in development and not polished.
As a prerequisite for this to be useful, we have to sync the appendix and the Coq sources again - at least the first part, describing the model. I am not sure yet whether we want the rest of the appendix in here as well. It's less related to the Coq development, but it's more what people are actually interested in. @janno suggested some kind of Hackathon, where we sit together and check that definitions actually match. That should probably happen after #4 (closed) is done. @swasey what do you think?