Update website

[duckdalbe]

The website is out of date (e.g. the discussion where to store headers, text/rfc822-part vs. first protected mime-part).

It should be updated, otherwise people might think that this project is dead and refrain from supporting it.

[dkg]

I think we should instead transfer the maintenance of this documentationt to the Autocrypt project (see autocrypt/autocrypt#134)

[varac]

http://modernpgp.org/memoryhole/specs/ times out recently.