Add documentation for kos-registry

[stefandesu]

kos-registry is now included as a git submodule, but there are no instructions that it is necessary to fetch that. So we need something like this:

# Either
git submodule init
git submodule update
# Or
git clone --recurse-submodules https://github.com/gbv/cocoda-mappings.git

[nichtich]

We should better remove the submodule and get kos-registry by other means (see https://github.com/gbv/kos-registry/issues/25 and https://github.com/gbv/kos-registry/issues/9).