this should address #27, #28 and #29 at least at a basic level
it is mostly based on the mailing list discussion mentionned in #27,
with some reformatting and merging of different posts.
this is still incomplete as it only describes key files, but doesn't
clearly say how chunks are encrypted or decrypted.
this address parts of #29 but eventually that document should also
cover #27, #28 and maybe #45
