docs: add NAT traversal related documentation

sample-lnd.conf

@@ -77,6 +77,18 @@
 ; (with host:port notation), the default port (9735) will be added to the
 ; address.
 ; externalip=
+;
+; Instead of explicitly stating your external IP address, you can also enable
+; UPnP or NAT-PMP support on the daemon. Both techniques will be tried and
+; require proper hardware support. In order to detect this hardware support,
+; `lnd` uses a dependency that retrieves the router's gateway address by using
+; different built-in binaries in each platform. Therefore, it is possible that
+; we are unable to detect the hardware and `lnd` will exit with an error
+; indicating this. This option will automatically retrieve your external IP
+; address, even after it has changed in the case of dynamic IPs, and advertise
+; it to the network using the ports the daemon is listening on. This does not
+; support devices behind multiple NATs.
+; nat=true
 
 
 ; Debug logging level.