Add a warning that Jami is partially centralized

[lrq3000]

Description

Although it is described as a P2P client, Jami uses 5 different central servers to provide some functionalities, as described here and here. Hence, without self-hosting, these servers are points of failure where meta-data can leak (see also #1357).

This by the way goes counter to the cited advantage of P2P clients: "There are no servers that could potentially intercept and decrypt your transmissions, unlike centralized and federated models."

I suggest a warning is added about the possibility to disable/self-host these servers.

[blacklight447]

I think small note is would be a proper solution to this. this issue is one of the grey areas which are always difficult to define. Do you wnna make a quick PR for this @lrq3000 ?

[lrq3000]

@blacklight447-ptio yes I'll do it :-)