Added links related to NAT config in a new FAQ doc

doc/faq.md

+**1. How to tell if my server instance is behind NAT?**
+
+A. In general, if the tool ifconfig (or ipconfig) shows the assigned IP address to be some local address (10.x.x.x or 192.x.x.x) but you know that its public IP address is different from that, the server is most probably behind NAT
+
+**2. Clients could communicate well in room created at meet.jit.si . The same clients still could connect to my self-hosted instance but can neither hear nor see one another. What's wrong?**
+
+A. Most probably, the server is behind NAT. See this [resolved question](https://community.jitsi.org/t/cannot-see-video-or-hear-audio-on-self-hosted-instance/). You need to follow the steps detailed [here](https://github.com/jitsi/ice4j/blob/master/doc/quick-install.md#Advanced-configuration)

doc/quick-install.md

 ```
 
 #### Advanced configuration
-If installation is on a machine behind NAT further configuration of jitsi-videobridge is needed in order for it to be accessible.
+If installation is on a machine [behind NAT](https://github.com/jitsi/jitsi-meet/blob/master/doc/faq.md) further configuration of jitsi-videobridge is needed in order for it to be accessible.
 Provided that all required ports are routed (forwarded) to the machine that it runs on. By default these ports are (TCP/443 or TCP/4443 and UDP 10000).
 The following extra lines need to be added the file `/etc/jitsi/videobridge/sip-communicator.properties`:
 ```