Add a small TURN read-me

TURN.md

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+# Setting up TURN/STURN
+
+If you run into problems while configuring Conduit to inform clients about TURN services, ask us in `#conduit:matrix.org` or [open an issue on GitLab](https://gitlab.com/famedly/conduit/-/issues/new).
+
+
+## General instructions
+
+* This was tested against Conduit(Next), Coturn (Archlinux) and Element (Android)
+* Build your own Conduit from source. It's dead simple to build.
+* Use the NEXT branch as only this branch provides the necessary functionality.
+* This documentation assumes you have a [Coturn server](https://github.com/coturn/coturn) up and runnig. See [here](https://github.com/matrix-org/synapse/blob/develop/docs/turn-howto.md) how to get it working with the reference implementation. It's almost the same with Conduit.
+
+## Add a few setting to your existing conduit.toml
+
+```
+# Coturn settings to inform the clients where to find a coturn server
+# to enable Audio/Video calls
+
+# domain.tdl should match the REALM setting of coturn
+# Adjust this as needed, If you have only one transport write it this way:
+# turn_uris = ["turn:domain.tld?transport=tcp"]
+turn_uris = ["turn:domain.tdl?transport=udp", "turn:domain.tld?transport=tcp"]
+
+# the value of static-auth-secret of your turnserver
+turn_secret = ""
+
+# the default
+turn_ttl = 86400 # 1 Day
+```
+
+## Apply setting
+
+Restart Conduit and enjoy audio and/or video call functionality from within Element.