README: suggested improvements for protocol flow diagram

[lumaier]

Some things I think are not really clear or wrong in the flow diagram (even tough it is really good overall):

• Right at the end where message_ciphertext is decrypted at the source, the order of the concatenated messages is different to how the journalist encrypted them ($reply || JC_{PK} ||{}^mJE_{PK}$)
• It looks like as if the server initiates the "fetch" operation (i.e., a "fetch" message sent to the server is missing IMO)
• When looking at the "read" operation at the journalist's side, it isn't really clear how $message\_id$ is derived (although one can guess that it is "the one" which was successfully decrypted-flow diagram doesn't cover the case when multiple ids are decrypted).
• The preceeding superscript ${}^{0-i}JE_{SK}$ makes sense in my opinion if you mean ${}^0JE_{SK},\dots,{}^iJE_{SK}$. Inside a "for" loop I would go for an index-based notation such as: $\text{for }j = 0,\dots,i: {}^jJE_{SK} = Gen()$
• I would add a comment in the flow diagram that a journalist tries all unused ephemeral keys ${}^iJE_{SK}$ when decrypting a ciphertext