Add TURN server Q.

supporting-docs/guides/2015-08-19-faq.md

@@ -486,6 +486,12 @@ Since events are extensible it is possible for malicious users and/or servers to
 
 Voice (and video) over Matrix uses the WebRTC 1.0 standard to transfer call media (i.e. the actual voice and video traffic).  Matrix is used to signal the establishment and termination of the call by sending call events, like any other event.  Currently calls are only supported in rooms with exactly two participants - however, one of those participants may be a conferencing bridge.  We're looking at better ways to do group calling.
 
+##### Do I need a TURN server?
+
+VoIP calls should work if both parties are on public networks. However, in practice one (or both) devices are often behind NAT, and so having a [TURN](https://en.wikipedia.org/wiki/Traversal_Using_Relays_around_NAT) server is important to help set up the call.
+
+See [this guide](https://github.com/matrix-org/synapse/blob/master/docs/turn-howto.rst) for setting up a TURN server with Synapse.
+
 ##### Can I log into other homeservers with my username and password?
 
 Currently, no. We are looking at options for decentralising or migrating user accounts between multiple servers, and might add this feature at a later stage.