FAQ: add a part about VOIP

That’s a common request those days. The entry should link to https://labs.riseup.net/code/issues/5709 (VoIP support) and list the common tools:

Jitsi -> no, this is dead
Linphone /SIP phones -> probably not “anonymous” if it works at all, as using a phone number
meet.jit.si and friends -> WebRTC does not work over Tor
Skype - to say it’s not supported

Original created by @BitingBird on 9232 (Redmine)

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