docs: add signaling sequence diagram

[WofWca]

I heard you'd like a diagram. Let me know if this is what you wanted to see, if this clears things up.

[link2xt]

The app also uses a WebRTC data channel. What is it used for? If someone wants to switch away from this application plus webview and implement calls natively (currently happens on iOS) they need to know what happens on this data channel or can it just be ignored? In any case even if it can be ignored at the cost of e.g. worse roaming between networks, the diagram currently does not show that it is used.

[WofWca]

I was hesitant to include this detail because apparently using a data channel for signaling is not great, i.e. we might do proper ICE trickling over email in the future. But OK, I'll include it.

[Amzd]

LGTM, lets merge this so the proposed data channel in #84 can be added to the diagram there instead of having to update this PR again.

[nicodh]

This diagram is a good improvement of the docs!

I agree that the part about the data channel should be a bit more verbose but that can be improved later