core: Add a comment header about the format
core: Add a comment header about the format

This should probably be a design document or something, but this is
useful for now.

sujitfulse/ostree,resin-io/ostree,resin-io/ostree,resin-io/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,leandrosansilva/ostree,gatispaeglis/ostree,GNOME/ostree,sujitfulse/ostree,sujitfulse/ostree,leandrosansilva/ostree,resin-io/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,gatispaeglis/ostree,leandrosansilva/ostree,leandrosansilva/ostree