Add Elixir WebRTC - Nexus app demo to docs

[Megzo]

I created a new demo description to showcase how to use STUNner with Elixir WebRTC. Please check if everything works correctly ;)

[rg0now]

Thanks, this is really cool! I think if we just fix the typos this can be merged as is and so it can make it into v1. I'll add Elixir to the release notes.

[levaitamas]

closing this in favor of #164